We examined the bacterial aerobic nasal flora of 216 healthy volunteers to identify potential competitive interactions among different species, with special emphasis on the influence of staphylococcal agr alleles. The Staphylococcus aureus colonization rate correlated negatively with the rate of colonization by Corynebacterium spp. and non-aureus staphylococci, especially S. epidermidis, suggesting that both Corynebacterium spp. and S. epidermidis antagonize S. aureus colonization. Most of the S. aureus and S. epidermidis isolates were agr typed by a PCR method. Only one S. aureus agr (agr(Sa)) allele was detected in each carrier. Multiple logistic regression of the two most prevalent agr(Sa) alleles (agr-1(Sa) and agr-2(Sa)) and the three S. epidermidis agr (agr(Se)) alleles showed a specific influence of the agr system. The results of this model did not support conclusions drawn from previous in vitro agr-specific cross-inhibition experiments. Our findings suggest that the agr alleles, which are strongly linked to the bacterial genetic background, may simply be associated with common biological properties--including mediators of bacterial interference--in the strains that bear them.