The paper describes the synthesis of the phosphorylcholine-binding miniantibody McPC603scFvDhl x in cell-wall-less L-form strains of Escherichia coli and Proteus mirabilis. Cells of these strains were transformed with the plasmid pACK02scKan, carrying the miniantibody (miniAb) coding sequence under the control of the lac promoter. L-form transformants of both species were able to synthesize the functional miniAb as an extracellular soluble product. The highest quantities were obtained by P. mirabilis L-form strains after induction with 5 mM isopropyl beta-D-thiogalactopyranoside (IPTG). Yields of 45-75 mg/l total antibody protein and of 10-18 mg/l functional miniAb were estimated in the growth medium of shaking cultures 40-80 h after induction with IPTG. About 10% of the active miniAb remained cell-bound. The yields of functional miniAb could be optimized by lowering the growth temperature from 37 degrees C to 26-32 degrees C and by supplementation of the medium with 80 mM sodium fumarate. A comparison of the specific activities revealed that the P. mirabilis L-form strains have a similar synthesis capacity (2-4 mg functional miniAb/g cell dry weight) to that of the producer strain E. coli RV308. The results show that the processes of correct folding and assembling of the miniAb molecules are possible without the periplasmic compartment.