The minimal inhibitory concentrations (MICs) of 18 antibiotics including 16 beta-lactam antibiotics were determined by agar dilution on 70 strains of pneumococci (25 penicillin sensitive, 18 intermediate resistant and 27 resistant). The antimicrobials tested were penicillin G, ampicillin, carbenicillin, ticarcillin, piperacillin, mezlocillin, cephalothin, cefoxitin, cefamandole, latamoxef (moxalactam), cefotaxime, cefoperazone, ceftazidime, ceftriaxone, N-formimidoyl thienamycin, SCH 29482, chloramphenicol and vancomycin. Of these agents, only cefotaxime demonstrated greater activity than penicillin against intermediate penicillin-resistant strains while cefoperazone, ceftriaxone, N-formimidoyl thienamycin and vancomycin as well as cefotaxime demonstrated activity superior to penicillin against penicillin-resistant strains. Comparison of the MIC90s of these agents with the achievable cerebrospinal fluid levels suggests that meningitis caused by penicillin-resistant pneumococci should respond to treatment with cefotaxime, cefoperazone, ceftriaxone and vancomycin.