The specific binding of radiolabeled polymyxin B (PmB) to rat alveolar macrophages was investigated. PmB retained its ability to inhibit lipopolysaccharide-induced tumor necrosis factor production by macrophages as long as one of five amino groups on PmB was unbound. Binding was saturable and temperature- and time-dependent, reaching steady state by 30 min at 37 degrees C and by 18 h at 4 degrees C. Macrophages had approximately 1.6 X 10(7) (Kd = 0.28 nM) PmB binding sites per cell. Lipid A had no appreciable effect on the number of sites. Binding did not occur to rat platelets, L929 fibroblast cells, a rat thymoma cell line, or precursor monocytic and myeloid cell lines. Precursor cells activated with 12-O-tetradecanoylphorbol-13-acetate acquired binding similar to that seen in alveolar macrophages, but L929 fibroblasts did not. Binding sites were sensitive to trypsin but not to phospholipase C. PmB may interact with specific binding sites involved in lipopolysaccharide-induced activation, production, or release of tumor necrosis factor by macrophages, inhibiting the effects of lipopolysaccharide on macrophages.