The susceptibility of male Swiss white mice (MF1) to Staphylococcus aureus-induced arthritis was investigated with wild-type strain allelic replacement mutants. Comparison was made with a known mouse arthritogenic strain. The development and severity of arthritis were dependent both on the numbers of live bacteria injected intravenously and also on the mutant used; the ID50 ranged from (5 X 10(6))-(1 x 10(8)) cfu. The results indicate that expression of the genes associated with virulence, including those for protein A and alpha-haemolysin, play a major role in the pathogenesis of staphylococcal septic arthritis. When either virulence component was carried by the S. aureus variant, a greater degree of inflammation, pannus formation and cartilage destruction was detected histologically. Loss of one or more virulence factors lowered the septic arthritis severity score based on clinical and histological parameters.