A competitive enzyme-linked immunosorbent assay (ELISA) for quantification of protein A produced by Staphylococcus aureus A676 (a methicillin-resistant strain) was based on competitive binding to rabbit IgG molecules between alkaline phosphatase-labelled protein A and unlabelled protein A. The optimum IgG concentration required for coating was 2-4 fig I ml. The optimum incubation time for colour development using (p-nitrophenol phosphate substrate was 20-30 min. The lowest protein A concentration that could be measured using the optimized competitive ELISA was 20 ng/ml, and the maximum 2 fig I ml. The amount of protein A produced in brain-heart infusion medium increased exponentially during log phase of cell growth, reaching a maximum concentration of 22.5 fig I ml after 15 h cultivation.