The synthesis of several exoproteins, including protein A (SpA) in Staphylococcus aureus is coordinately regulated by the agr locus. Different constructs of the SpA-encoding gene (spa) were introduced into Agr+ and Agr- derivatives of a spa- strain of S. aureus. Plasmid-located spa with deletions at the 3' end expressed a truncated SpA which was almost exclusively extracellular and which confirmed the role of C-terminal region X in cell-wall binding. In the Agr- host, the production of SpA was elevated severalfold. Transcriptional and translational fusions were constructed to study the agr- mediated regulation of spa gene expression. Translational fusions of a beta-lactamase (Bla)-encoding ApR reporter gene with the spa promoter and N-terminal coding sequences expressed elevated levels of Bla activity in the Agr- host. In contrast, a transcriptional fusion of the spa gene with a promoter of the positively regulated staphylococcal epidermolytic toxin A (ETA)-encoding gene synthesized higher levels of SpA in an Agr+ host, as compared to Agr-. Moreover, the synthesis of SpA in the Agr+ strain was switched on during the transition from the exponential to stationary phase in a similar fashion to ETA itself. These data strongly indicate that the regulation of both SpA and ETA occurs at the transcriptional level in S. aureus. The agr-regulated spa promoter was defined by deletion analysis and by transcript mapping.