This in vitro study of thiamphenicol activity against 247 strains of Gram-positive and 195 strains of Gram-negative pathogenic bacteria showed no significant changes from past activity. Of the 442 strains involved, thiamphenicol proved: - Effective (MIC = less than or equal to 0.5-4 micrograms/ml) against 131 strains, mostly streptococci, pneumococci and enterococci. - Moderately effective (MIC = 8-64 micrograms/ml) against 252 strains, mostly staphylococci, Salmonella, Citrobacter, K. pneumoniae, Enterobacter, E. coli and Proteus. - Ineffective (MIC = 64 micrograms/ml) against 59 strains, mostly Proteus, E. coli and K. pneumoniae. Against 23 strains of Neisseria gonorrhoeae, thiamphenicol generally proved as effective as penicillin G and cefuroxime and more effective than spectinomycin.