Following the coincidental discovery that beta-actin isolated from renal epithelial cells was precipitated by staphylococcal protein A (SPA), the possibility that SPA and cytoskeletal actin filaments may be involved in Staphylococcus aureus infection of epithelial cells was considered. Therefore, to clarify the potential role of SPA and actin filaments in S. aureus infection, the invasion efficiency of S. aureus was determined quantitatively by measuring the number of cfu of viable organisms recovered from cultured KB cells. S. aureus invasion was found to be time dependent (0-60 min) and increased linearly when increasing numbers of bacteria were added (10(4)-10(6) cfu/ml). However, significant variation in the level of invasion was noted in protein A-deficient S. aureus Wood 46. Cytochalasin B inhibited the invasion efficiency of S. aureus in a dose-dependent manner. The present study suggests that interaction of staphylococcal protein A and cytoskeletal actin filaments is involved in the S. aureus invasion of cultured KB cells, and this process may contribute, in part, to the intracellular movement, cell-to-cell spread and dissemination of S. aureus within human oral epithelial cells in vivo.