Group B streptococcal isolates (n=159) from the three Dublin maternity hospitals, were serotyped and analysed for the bac, bca, hylB, pepB, and rib genes. The serotype distribution of the isolates was Ia, 19·5%; Ib, 18·9%; II, 10·7%; III, 29·5%; IV, 1·9%; V, 15·1%; non-typeable, 4·4%. There was a statistically significant association between the serotype and invasive status (carriage or infection) of isolates (P<0·005), but no significant association between serotype and degree of invasiveness was demonstrated. The presence or absence of each analysed gene was not associated with the invasive status of isolates. Statistically significant associations were revealed between bca and hylB (IS1548) (P=0·0004) and between bac and bca (P=0·0l4). The bac, bca, hylB (IS1548) and rib genes and the numbers of tandem repeats in the bca gene showed significant associations with serotype. Almost 50% of serotype III isolates possessed at least one of the bac and bca genes and 55–65% of strains of serotypes Ia, Ib and II possessed the rib gene. Most serotype III isolates had IS1548 in their hylB genes. Serotype Ib was the only serotype in which more than half of the strains contained more tandem repeats in the bca gene than the overall mean for the GBS population studied of 7·4 repeats. These findings indicate that some previously reported associations between putative virulence factors and GBS disease require further study and clarification.