In order to characterize serum‐resistant and serum‐sensitive strains ofN. gonorrhoeae, the protein I serotype, auxotype, and penicillin susceptibility of 128 strains were tested. Sensitivity to the complement‐dependent bactericidal activity of normal human serum was highly associated with protein I serotype (p<0.001). Thus 85% of serotype 1–3 strains were serum‐resistant, whereas 86% of serotype 8 strains and all strains with serotypes 8 + 9 or 9 were serum‐sensitive. Serum‐resistance or sensitivity for a given serotype was independent of auxotype. The susceptibility to penicillin within the serotypes 1 – 3 was significantly associated with auxotype (p = 0.0016); all AHU‐ (requirement for arginin, hypoxanthine and uracil) strains had MICs of penicillin of 0.04 μg/ml or less and were serotypes 1 – 3. Among the non‐AHU‐strains, serotype 9 was significantly more penicillin susceptible than the o