We have determined that isolates of Neisseria gonorrhoeae from patients with disseminated gonococcal infection (DGI) are different from randomly collected isolates from patients with uncomplicated (local) disease. Our comparison was based on the six phenotypic properties of: sensitivity to penicillin (PenS), erythromycin, and streptomycin; resistance to the bactericidal effects of pooled human sera; requirements for arginine, hypoxanthine, and uracil (AHU-); and sensitivity to toxic agar. Although the marked association among these traits made analysis difficult, several factors independently related to virulence were defined. The DGI isolates were significnatly more PenS and resistant to serum, even when the other variables were held constant. An apparent correlation between AHU- auxotype and virulence was shown to be due to the PenS property of most AHU- isolates. Thus, certain mutations to antibiotic resistance as well as susceptibility to sera, may result in loss of virulence in the gonococcus, perhaps through alteration of cell envelope structure.