An aroA mutant of gonococcal strain MS11 was constructed (JKD298) and compared with the wild-type (JKD288). The requirement of JKD298 for aromatic compounds, typical of an aroA mutant, was demonstrated using defined media. Other than the expected auxotrophy, no further differences could be demonstrated between the parent and the aroA mutant. SDS-PAGE analysis of protein and lipopolysaccharide (LPS) profiles showed no differences between the strains. Bactericidal assays using human and guinea-pig normal sera showed that both strains were serum sensitive and were similarly converted to serum resistance by in vitro sialylation using CMP-NANA. Infectivity experiments in guinea-pig subcutaneous chambers showed considerably reduced virulence of the aroA JKD298, which could only infect chambers at very high doses. Established infections by either strain elicited a strong PMN response in which similar proportions of each strain were seen intracellularly. Infections by JKD298 provoked a strong antibody response as detected by ELISA using whole sonicated gonococci. This is the first demonstration of attenuation of Neisseria gonorrhoeae by introduction of a defined mutation in a metabolic gene.