The effect of prednisolone and non-steroid anti-inflammatory drugs on PMNL (polymorphonuclear leucocytes) oxidative metabolism was quantified with a newly standardized NBT test, and it was investigated whether these effects correlate with phagocytosing capacity of PMNL. Prednisolone inhibits NBT reduction in dose dependency already at concentrations, which do not interfere with phagocytosis. Thus prednisolone dissociates phagocytosis and phagocytosis-associated oxidative metabolism. High doses of prednisolone also inhibit phagocytosis. These effects of prednisolone are still demonstrable when PMNL are washed after pre-incubation with the drug. The non-steroid anti-inflammatory drugs (indomethacin, phenylbutazone and pyrazinobutazone) inhibit phagocytosis and NBT reduction at equivalent doses. When PMNL are washed after incubation with the drugs, they regain normal capacity to phagocyse and to reduce NBT. It is suggested that these drugs inhibit phagocytosis directly, and consequently the phagocytosis-associated oxidative metabolism is suppressed.