Staphylococcus aureus is a major human pathogen that produces many virulence factors in a temporally regulated manner controlled by at least two global virulence regulatory loci (agr and sarA). We identified previously a two-component system, ArlS-ArlR, that modifies the activity of extracellular serine protease and may be involved in virulence regulation. Here, we show that mutations in either arlR or arlS increase the production of secreted proteins [alpha-toxin (Hla), beta-haemolysin, lipase, coagulase, serine protease (Ssp)] and especially protein A (Spa). Furthermore, the pattern of proteins secreted by both mutants was strikingly different from that of the wild-type strain. Transcriptional fusions showed that expression of hla, ssp and spa was higher in both mutants than in the wild-type strain, indicating that the arl operon decreases the production of virulence factors by downregulating the transcription of their genes. The arl mutation did not change spa expression in an agrA mutant or in a sarA mutant, suggesting that both the sarA and the agr loci are required for the action of arl on spa. Northern blot analyses indicated that the arl mutation increased the synthesis of both RNA II and RNA III, but decreased sarA transcription. Finally, arl was not autoregulated, but its expression was stimulated by agr and sarA. These results suggest that the Arl system interacts with both agr and sarA regulatory loci to modulate the virulence regulation network.