Unresolved experimental Lyme arthritis in C3H (5-LOX)(-/-) mice is associated with impaired macrophage phag-ocytosis of Borrelia burgdorferi. In the present study, we further investigated the effects of the 5-LOX metabolite, (LT)B-4, on phagocytosis of B. burgdorferi. Bone marrow-derived macrophages (BMDM) from 5-LOX-/- mice were defective in the uptake and killing of B. burgdorferi from the earliest stages of spirochete internali-zation. BMDM from mice deficient for the LTB4 high-affinity receptor (BLT1(-/-)) were also unable to efficiently phagocytose B. burgdorferi. Addition of exogenous LTB4 augmented the phagocytic capability of BMDM from both 5-LOX-/- and BLT1(-/-) mice, suggesting the low-affinity LTB4 receptor, BLT2, might be involved. Blocking BLT2 activity with the specific antagonist, LY255283, inhibited phagocytosis in LTB4-stimulated BLT1(-/-) BMDM, demonstrating a role for BLT2. However, the lack of a phagocytic defect in BLT2(-/-) BMDM suggested this was a compensatory effect. In contrast, 12-HHT, a natural BLT2-specific high-affinity ligand, and RvE1, a BLT1 agonist, were both unable to boost phagocytosis in BMDM from either 5-LOX-/- or BLT1(-/-) mice, suggesting a specific role for LTB4 in mediating phagocytosis in murine macrophages. This study demonstrates LTB4 promotes macrophage phagocytosis of bacteria via BLT1, and that BLT2 can fulfil this role in the absence of BLT1.