Marek's disease is a common lymphoproliferative disease of chickens, usually characterized by mononuclear cell infiltration of peripheral nerves and various other organs and tissues, including the iris and skin. A group of 70 newly-hatched chicks was vaccinated by means of nebulization and exposed for 60 seconds to the HVT FC 126 vaccine, while the other group of 70 chicks received the same vaccine by parenteral route (s.c.). The aim of this study was to compare the morphological patterns of the chicken's lymphocytes in their peripheral blood, before and after vaccination by means of either nebulization or parenteral injection. Image analysis was performed using the SFORM software (VAMSTEC, Zagreb, Croatia). A total of 50 blood smears from vaccinated chickens (20 by means of nebulization and 30 by parenteral injection), with an average of 100 cells per smear, were analyzed. The results showed that the peripheral blood lymphocytes of chickens vaccinated by means of nebulization, compared with parenteral vaccinated, had significantly higher values for the majority of the measured variables: area, outline, minimum and maximum radius, length, breadth and convex area, on all days after vaccination, except on day 4, indicating the significantly higher metabolic activity of those cells. In lymphocytes of chickens vaccinated parenterally, only the nucleo-cytoplasmic ratio was higher. The results show that morphological patterns of immunocompetent cell transformation could be used to evaluate immune responses to vaccination and vaccine efficacy. We conclude that nebulization as a mode of vaccination against Marek's disease stimulates the transformation of immunocompetent cells much earlier, thus shortening the time of immunosuppression and improving the immune response. This is of paramount importance for the practical application of Marek's disease vaccine. Key words: chickens, Marek's disease, nebulization, parenteral vaccination, lymphocyte morphometry Marekova je bolest limfoproliferativna zarazna bolest kokosi, obicno karakakterizirana mononuklearnim stanicnim infiltacijama u perifernim zivcima i ostalim organima i tkivima, ukljucujuci sarenicu i kozu. Skupina od 70 netom izleglih pilica cijepljena je protiv Marekove bolesti cjepivom od soja FC 126 herpesvirusa purana postupkom nebulizacije izlaganjem aerosolu tijekom 60 sekundi, dok je druga skupina od 70 pilica primila isto cjepivo parenteralnim nacinom (supkutano). Cilj ovog istrazivanja bio je usporediti morfometrijske znacajke limfocita u perifernoj krvi pilica prije i nakon cijepljenja kako postupkom nebulizacije tako i parenteralnim nacinom primjene. Kompjutorska analiza slike ucinjena je na osobnom racunalu koristenjem programa "SFORM" (VAMSTEC, Zagreb, Hrvatska). Ukupno je pretrazeno 50 obojenih razmaza periferne krvi cijepljenih pilica (20 postupkom nebulizacije i 30 parenteralno), te je analizirano prosjecno 100 limfocita po uzorku. Rezultati su pokazali da su limfociti u perifernoj krvi pilica cijepljenih postupkom nebulizacije u odnosu na parenteralno cijepljene bili znacajno veci u vecini istrazivanih pokazatelja: povrsini, opsegu, minimalnom i maksimalnom polumjeru, duzini i sirini, te ispupcenosti odnosno konveksnosti povrsine, u svim danima pokusa osim cetvrtoga, sto ukazuje na pojacanu metabolicku aktivnost tih stanica. U limfocitima parenteralno cijepljenih pilica samo je omjer jezgre i citoplazme bio veci. Postignuti rezultati pokazuju morfoloske znacajke transformacije imunokompetentnih stanica, koje se mogu rabiti u procjeni imunosnog odziva na cjepivo, ucinka cijepljenja i moguceg docjepljivanja. Zakljucili smo da nebulizacija kao metoda cijepljenja protiv Marekove bolesti stimulira transformaciju imunokompetentnih stanica mnogo ranije, skracujuci time vrijeme imunosupresije i pojacavajuci imunosni odziv. To je od osobite vaznosti za prakticnu primjenu cjepiva protiv Marekove bolesti. Kljucne rijeci : pilici, Marekova bolest, nebulizacija, parenteralno cijepljenje, morfometrija limfocita