The biological properties of a series of opacity variants of Neisseria gonorrhoeae P9 have been examined. A novel protein, designated protein IId* (mol. wt 28 850), was identified within the set of heat-modifiable surface proteins previously reported. All variants producing extra outer membrane proteins were less sensitive to the bactericidal action of serum than the prototype transparent strain, with protein IIa* (mol. wt 28 500) being associated with increased resistance. The production of a different protein, protein II* (mol. wt 29 000), was correlated with resistance to low molecular weight antimicrobial agents (penicillin, fusidic acid, Cu2+, Zn2+). Increased adhesion to human buccal epithelial cells was demonstrated in all variants that produced extra surface proteins. These variants did not show increased binding to hexyl- and phenyl-substituted Sepharose gels suggesting that hydrophobic interaction was not responsible for their cohesive properties. The prototype strain lacking additional proteins demonstrated the greatest binding to erythrocytes, indicating that adhesion to buccal cells and red blood cells is mediated by different mechanisms. One variant producing protein IIa* showed increased association with leukocytes, whereas another producing protein IIb* showed decreased association with leukocytes. These results show that the heat-modifiable surface proteins are important virulence attributes of the gonococcus: this must be considered in the selection of strains for vaccine trials.