Spheroplasts were obtained from Proteus vulgaris, in sinthetic culture media, having Penicillin as the inductor agent, in different concentrations. Once stablished the optimum dosage of sensibility of the bacterium, to the antibiotic to obtain "large bodies" from the "L" Cicle, morphologically defined, as spheroplasts, nuclear masses, in several stages of the bacterial growth, were stained. The material was collected by cover-slip impression, using osmic acid as fixative, with heated cloric hydrolysis, and staining by fuchsin. From the obtained results it became evident: 1 - the possibility of obtaining "large bodies" from Proteus vulgaris in a simple sinthetic culture medium, composed basically of mineral salts and glucose; 2 - that there existed an optimum concentration of Penicillin to induce the formation of spheroplasts, in the logarithmic growth fase; 3 - the ease by which the nuclear masses stain in the spheroplasts in several periods of growth; 4 - the reversion to the normal morphological type, after 24 hours of growth in culture medium containing the antibiotic; 5 - changes in the growth of Proteus vulgaris in agar surface, with a minimum Penicillin dose, forming localized colonies.