1. Binding of [3H]-leukotriene B4 ([3H]-LTB4) to murine spleen membranes (MSM) was determined. 2. Scatchard analyses of [3H]-LTB4 binding indicated the presence of high (KD1 = 1.7 nM) and low (KD2 = 7.5 nM) affinity receptors on MSM with Bmax values of 151 fmol mg-1 protein (Bmax1) and 354 fmol mg-1 protein (Bmax2), respectively. 3. CP-105,696, a potent LTB4 antagonist, inhibited [3H]-LTB4 (0.67 nM) binding to the high affinity receptor on MSM, IC50 = 30.2 nM, Ki = 17.7 nM with a Hill coefficient of 0.93. 4. Scatchard analyses of [3H]-LTB4 binding to MSM in the presence of CP-105,696 indicated that the high-affinity receptor was inhibited in a non-competitive manner and the low-affinity receptor in a competitive manner. 5. Isolated peripheral blood murine neutrophils (MN) responded chemotactically to LTB4, EC50 = 2.5 nM. CP-105,696 blocked this response, IC50 = 2.3 nM. When examined over a full concentration-response range of LTB4, CP-105,696 inhibited chemotaxis in a non-competitive manner. 6. Murine neutrophils in anticoagulated whole blood upregulated the integrin, complement receptor type 3 (CD11b/CD18, Mac-1) in response to LTB4, EC50 = 20 nM and this was inhibited by CP-105,696 in a competitive manner. 7. These results provide evidence that MSM have specific binding sites for LTB4, and as exemplified by CP-105,696, that these receptors may be useful for determining the potency and nature of antagonism of novel LTB4 receptor antagonists.