Compounds having a-[dihydro-5-substitted 6-thioxo-2H-1.3.5-thiadiazine-3(4H)-yl]benzylpenicillin structure were synthesized by the reaction of ampicilliin trihydrate, formaldehyde and dithiocarbamic acid salts. The smctures were evident from chemical and spectral analysis. The antimicrobial activities of the compounds were investigated against some gram-positive (Staphylococcus aweus and Streptococcus faecalis) and gram-negative (Escherichia coli and Pseudomom aeruginosa) bacteria and some yeastlike fungi (Candi& albicans, C. parapsilosis, C. stellatoidea and C . pseudotropicalis) and molds such as Trichophyton rubrum, T. mentagrophytes, Microsporum canis. M. gypseum, Penicillium and Aspergillus by the tube dilution method. In addition to MIC (minimal inhibitory concentration), MBC (minimal bactericidal concentration) and MFC (minimal fungicidal concentration) values were determined using ampicillin hihydrate as standard. The compounds synthesized were usually found as effective as ampicillin trihydrate against S. aweus and S. faecalis and less effective than ampicillin trihydrate against E. coli. Both the compounds synthesized and ampicillin trihydrate are ineffective in the concentrations studied against P. aeruginosa. Compound 10 and 11 are more effective against all the yeast-like fungi than the other compounds and ampicillin trihydrate.