The ability of staphylococci adherent to silicone surfaces to induce superoxide anion (O2-) production by polymorphonuclear leukocytes (PMNs) was investigated and compared with the same activity induced by planktonic bacteria. The responses to Staphylococcus aureus strain E 2371 and Staphylococcus epidermidis strain ATCC 14990 were compared. The staphylococci were allowed to adhere to silicone catheters for 2 h at 37 degrees C. After opsonization of adherent bacteria in 30% human AB-positive serum, the induction of superoxide anion production by PMNs was measured in a cytochrome C reduction assay. Both bacterial strains, when adhered to the surfaces, were able to induce superoxide anion production by PMNs to about the same extent. Comparing adherent and planktonic bacteria with these two bacterial strains, it was found that planktonic S. epidermidis induced one to three times higher superoxide anion production than the adherent bacteria, whereas planktonic S. aureus induced four to seven times higher superoxide anion production than the adherent bacteria. Interstrain variation between the response to adherent and planktonic staphylococci was found. The lower phagocytic response to adherent staphylococci as compared to the response to planktonic organisms may interfere with the killing process and thereby contribute to poor clearance of these bacteria when adherent to foreign bodies such as catheters.