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Eleven new 7-[2-(dihydro-5-substituted-6-thioxo-2H-1,3,5-thiadiazine-3( 4H)-yl)-2- (4-hydroxyphenyl)acetamido]-3-methyl-3-cephem-4-carboxylic acid derivatives were synthesized by the reaction of cefadroxil monohydrate, formaldehyde and substituted potassium dithiocarbamate. Their structures have been elucidated by spectral data and elementary analysis. The title compounds were tested for antimicrobial activity in vitro against gram-positive bacteria (Staphylococcus aureus, Streptococcus faecalis), gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) and yeast-like fungi (Candida albicans, C. parapsilosis, C. stellatoidea, C. pseudotropicalis) in comparison with cefadroxil monohydrate. The activity of compounds 1 and 10 against S. aureus (MBC: 37.5 micrograms/ml) and compound 1 against E. coli (MBC: 75 micrograms/ml) were found to be the same as cefadroxil monohydrate. Compounds 1 and 10 were more effective than cefadroxil monohydrate against S. faecalis with 25 and 37.5 micrograms/ml MBC values, respectively. None of the compounds and cefadroxil monohydrate proved to be effective against P. aeruginosa (MBC: greater than 100 micrograms/ml). While cefadroxil monohydrate had no activity against yeast-like fungi, compounds 9 and 10 were significantly effective against yeast-like fungi (MFC: 37.5 micrograms/ml).