The in vitro activity of 8 cephalosporins – cephalothin, cefamandole, cefoxitin, cefuroxime, cefotaxime, cefoperazone, moxalactam and ceftriaxone (Rocephin®) was studied on 109 strains of Neisseria gonorrhoeae and 60 strains of Neisseria meningitidis, isolated from pathological material. Determination of the MICs of these antibiotics by an agar dilution method shows that on N. gonorrhoeae ceftriaxone (geometric mean of the MICs: 0.0008 μg/ml) is the most active, followed by cefotaxime (0.001 μg/ml); cefoperazone (0.008 μg/ml) and moxalactam (0.01 μg/ml) are one-tenth as active; then come cefuroxime (0.05 μg/ml), cefamandole (0.10 μg/ml) and finally cephalothin (0.26 μg/ml) and cefoxitin (0.26 μg/ml). The least susceptibility to penicillin and, to a greater degree, the production of β-lactamase (8 strains) affect the level of susceptibility to these cephalosporins, but the MICs always remain rel