Ezeject is a plungerless syringe prefilled with lyophilized measles vaccine. Ezeject syringes were compared with standard 3-cc syringes and 10-dose measles vaccine vials in the vaccination of 884 Guatemalan infants 8-23 months of age. Vaccination was performed by experienced vaccinators and by individuals without prior vaccination experience who received 2.5-3 hours of training. The overall seroconversion rate following administration was 96%, regardless of the type of syringe used or of the experience of the vaccinator. No significant adverse events were observed in children vaccinated with the new syringes. Although incomplete emptying was noted in 87% of the Ezeject syringes used, this had no effect on the serologic response to measles vaccine. Aspiration for detection of blood before injection of the vaccine was performed significantly (P < .001) less frequently with Ezeject than with 3-cc syringes by both experienced and inexperienced personnel. Inexperienced vaccinators administered measles vaccine significantly faster (P < .001) with Ezeject than with 3-cc syringes, but the times were similar for experienced vaccinators. Ezeject is an acceptable alternative to standard syringes for the administration of measles vaccine. Several design modifications that would improve the handling of the device and eliminate the possibility of its reuse have been suggested.