In this study measurements were made of the minimal inhibitory concentrations of 6 cephalosporins (cefamandole, cefuroxime, cefadroxil, cefoxatin and cefotaxime in comparison with cefalothin) against 532 bacterial strains isolated at the Regional Hospital Centre, Nantes, France, between 15.12.1977 and 15.3.1978. There were 163 Gram-positive bacteria of 2 species and 369 Gram-negative bacilli of 11 species and different genera. The results have been produced in the form of cumulative curves, in percentage for the most numerous genera or species. Cefalothin remained the most active on S. aureus and Listeria. Cefamandole came second. The action of the four other cephalosporins was very closely related to that in relation to S. aureus. For all the Gram-negative bacilli, Cefotaxime was the most active. Its MIC's can be up to more than 100 times lower than those of the other cephalosporins. Of the others, cefamandole appeared the best. It was difficult to classify the last four, as the MIC's may vary from one bacterial species to another. The bacterial activity (MBC) in a liquid medium was found for 178 of the strains (10 genera or species) of which one was Gram-positive (S. aureus). The ratio MBC/MIC (the measurements were taken in a liquid medium in all cases) confirmed the fact that all the cephalosporins are clearly bactericidal (the ratio was most often equal to 1 and sometimes to 2) for all the Gram-negative bacilli. Cefalothin and cefotaxime appeared the most bactericidal and cefamandole the least.