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Staphylococcus aureus is a major human pathogen causing a wide range of illnesses from the trivial to the life-threatening. S. aureus produces many surface-associated and exoproteins, several of which have been implicated in its virulence. Production of these virulence determinants is co-ordinately controlled by several global regulatory elements in a growth phase dependent manner. The best characterised of these regulators are the accessory gene regulator (agr) and staphylococcal accessory regulator (sar). The agr locus comprises a quorum sensing system and encodes a signalling pheromone that autoregulates agr in a density dependent manner. Upregulation of agr expression leads to production of an mRNA transcript, RNAHI which is the actual effector of virulence gene expression. The RNAIII molecule upregulates several extracellular toxins including haemolysins, toxic shock syndrome toxin 1 (TSST-1) and epidermolytic toxin A (Eta), and down-regulates surface proteins such as protein A and fibronectin binding protein (FnBP) during late exponential growth and stationary phase. The regulation of toxin production by S. aureus is extremely complex and it is not yet understood exactly how this organism responds to environmental stimuli in order to mediate changes in virulence gene expression. In order to determine whether environmental signals are transduced via agr, the effect of several stimuli on both agr expression and a-haemolysin production was examined using a ß-galactosidase reporter gene fusion to the hid gene, which is encoded as part of the RNAIII transcript. A number of environmental stimuli were identified which led to changes in agr expression. Several of these stimuli resulted in different effects on a- haemolysin activity when compared to RNAIII levels. This suggests the presence of novel regulatory elements involved in the control of Hla production, independently of agr. In order to identify other novel regulators which interact with, or control, agr, transposon libraries have been created using Tn917 and Tn551. Two Tn917 transposon mutants were isolated as deficient in production of ß-haemolysin, which is also positively controlled by agr. These mutants were found to contain novel transposon insertions in the agr locus. Five Tn551 mutants were isolated which showed pleiotropic effects on virulence determinant levels and did not contain the transposon in previously mapped regulators. The Tn551 insertions may have therefore occurred in novel regulators of virulence determinant production. The regulation of toxin production by S. aureus in response to environmental stimuli is discussed.