Abstract Pulmonary sequestration and activation of polymorphonuclear leukocytes (PMNLs) are characteristic of many forms of acute lung injury. The present experiments were designed to study the effects of mepacrine on human neutrophils challenged with N-formylmethionyl- leucyl-phenylalanine (fMLP). Mepacrine inhibited fMLP-induced superoxide production and degranulation in a dose-dependent manner with Kd values of 2.3 ± 0.5 × 10−7 M and 5.7 ± 1.3 × 10−6 M, respectively. Stimulation of PMNLs by 10−6 M fMLP provoked the formation of barely detectable amounts of leukotriene B4 (LTB4) (< 5 pg/107 cells). Pretreatment of the cells with cytochalasin B augmented generation of LTB4 in response to fMLP (339 ± 79 pg/107 cells). LTB4 formation was also inhibited by mepacrine (50% inhibitory concentration 1.0 ± 0.5 × 10−6 M). Furthermore, mepacrine inhibited the specific binding of [3H]fMLP to neutrophils with a Ki value of 1.4 ± 0.4 ˟ 10”5 M. Mepacrine decreased the receptor binding affinity without altering the number of receptors. These findings demonstrate that the inhibitory effect of mepacrine is response dependent and suggest that this action of mepacrine could, in part, be attributed to a decrease in fMLP receptor affinity.