Human blood polymorphonuclear (PMN) leucocytes and human leucocytes of the HL-60 line, which were induced to differentiate by 1,25-dihydroxyvitamin D3, express stereospecific receptors for the potent chemotactic mediator, leukotriene B4 (LTB4), that is derived by 5-lipoxygenation from arachidonic acid. Monoclonal antibodies to LTB4 receptors (LTB4-R) were generated by immunizing BALB/c mice with partially purified PMN leucocyte membrane proteins, and fusing their splenocytes with P3X63Ag8 mouse myeloma cells. Hybridoma supernatants were screened initially by binding to PMN leucocyte LTB4-R protein, which had been affinity cross-linked with aminopropylamide (APA)-LTB4 and immobilized in plastic wells through attachment of the linked APA-LTB4 to adherent Fab of monoclonal anti-LTB4. Of the three clones producing antibodies which bound to LTB4-R, 0.5 mg/ml of one IgG3k antibody, termed E2, precipitated over 90% of the [3H]LTB4-binding activity of solubilized PMN leucocyte membrane proteins. E2 also bound to a radiolabelled protein of 70,000-80,000 MW from 125I-labelled PMN leucocyte membranes [35S]-labelled HL-60 cell membranes, and PMN leucocyte membranes affinity-labelled with [3H]APA-LTB4, that was identical in size to the LTB4-R precipitated by the rabbit IgG anti-idiotypic antibodies. E2 did not bind to intact PMN leucocytes or modify the binding of [3H]LTB4 by PMN leucocytes. The binding of E2 to LTB4-R in purified membranes of PMN leucocytes was less than one-fourth of that observed for the anti-idiotypic antibodies, but increased substantially after solubilization of the LTB4-R. The E2 monoclonal antibody thus recognizes a partially latent substituent of LTB4-R, which does not contribute to combining site function.