In vitro susceptibilities of 1478 strains of various pathogens isolated from clinical materials in 1981 to 23 antibiotics were studied using Showa disk diffusion test. Prevalence of bacterial resistance to antibiotics was evaluated. New cephem antibiotics such as cefmetazole (CMZ), cefoxitin (CFX) and cefotiam (CTM) had no increased activity over old cephalosporins such as cephalothin (CET) and cefazolin (CEZ) against Gram-positive cocci. However, all the new ones showed greater activities, broadened spectra, and/or both against Gram-negative bacilli, each offering unique advantages. S. aureus: Susceptible strains (MICs less than 15 micrograms/ml) to CET, CEZ, CTM, CFX and CMZ were 95%, 69%, 92%, 88% and 99%, respectively. Prevalence of resistance to CEZ was greater than to other cephalosporins, showing bimodal distribution of MICs. This may be due to more use of this drug in the past. Sensitive strains to benzylpenicillin (PCG), ampicillin (ABPC), sulbenicillin (SBPC) and piperacillin (PIPC) were 22 to 63%. A striking bimodal distribution of MICs of these penicillins was characteristic. Sensitive strains to various aminoglycosides and tetracyclines were 52 approximately 93% and 97 approximately 100%, respectively. S. pyogenes: 83 approximately 100% of strains were susceptible to various penicillins, CET and CEZ, whereas susceptible strains to CTM, CFX and CMZ were 66 approximately 90%. S. pneumoniae: Almost all strains (99%) were susceptible to PCG at the level less than 0.2 units/ml (0.12 microgram/ml). All strains were susceptible to various penicillins, cephalosporins, and cephamycins studied at the level less than 3 micrograms/ml. E. coli, K. pneumoniae and Proteus spp.: CTM, CFX and CMZ showed greater activity than CET and CEZ. Susceptible strains to the former antibiotics at the level less than 15 micrograms/ml were 93 approximately 94%, 82 approximately 95%, and 63 approximately 90%, respectively. Those to CET and CEZ were 61 approximately 83%, 85 approximately 92% and 53 approximately 58%, respectively. Among these cephalosporins, CMZ and CTM were the most effective, showing the least prevalence of resistance to these pathogens. H. influenzae: Susceptible strains (MICs less than 15 micrograms/ml) to ABPC, PIPC, CTM, CFX and CMZ were 80%, 81%, 84%, 55% and 71%, respectively. The majority of resistant strains to these beta-lactam antibiotics were sensitive to chloramphenicol, minocycline, doxycycline and erythromycin. P. aeruginosa, S. marcescens, E. aerogenes, Citrobacter and Achromobacter: CTM, CMZ and CFX were not effective against these pathogens.