We previously demonstrated that neutrophil (PMN) phagocytosis of opsonized zymosan (OPZ) caused oxygen-dependent inhibition of chemotactic peptide receptor (CPR) binding using the ligand 3H-formyl- methionyl-leucyl-phenylalanine (3H-FMLP). In the current studies we sought to determine the mechanism of CPR inhibition by OPZ. We found that 3 mM cysteine and 5 mM dithiothreitol (DTT) did not decrease PMN phagocytosis, but abolished CPR inhibition by OPZ. Furthermore, incubation of PMN with DTT after OPZ partially restored the suppressed CPR. PMN CPR saturation studies with 3H-FMLP on PMN incubated with or without DTT after phagocytosis indicated that DTT restored receptor numbers to 92% +/- 6% of prephagocytosis values and also improved CPR affinity for 3H-FMLP. In additional studies we found that the cell penetrating thiol reagents mercuric chloride (HgCl2), N-ethylmaleimide (NEM), and diamide, but not the nonpenetrating agents p- chloromercuriphenylsulfonic acid and p-chloromercuribenzoate, caused rapid, dose-dependent, DTT-inhibitable suppression of up to 85% CPR binding. CPR inhibition by HgCl2 and NEM was irreversible. PMN CPR saturation studies showed that NEM decreased total receptor numbers, not CPR affinity. The effect of NEM was not inhibited by receptor occupancy at 4 degree C or at 37 degree C. These studies indicate that: (1) intact thiol groups are a requirement for CPR ligand-binding; (2) the thiol groups are presumptively located on the interior side of, or within hydrophobic portions of, the plasma membrane and are not part of the 3H-FMLP binding site; (3) thiol oxidation-reduction induces reversible alteration in CPR binding; (4) phagocytosis-induced modulation of the CPR is due, in part, to thiol oxidation. These studies suggest that postphagocytic, thiol-mediated reduction in CPR binding may play a physiologic role in the inhibition of PMN chemotaxis at inflammatory sites. The state of CPR-associated thiols may be an important determinant of CPR binding under physiologic conditions other than phagocytosis.