The relative ratios of MIP-1α to MIP-1β released in response to LPS stimulation are different for in vivo versus in vitro models of endotoxemia. Circulating MIP-1α and MIP-1β levels following administration of a sublethal dose of LPS in (A) BALB/c and (B) BALB/cnu/nu mice. Mice were treated with LPS by intraperitoneal (ip) injection, and blood sampled at designated intervals. Quantification of MIP-1α (open circles) and MIP-1β (filled circles) in serum samples were determined by specific ELISA assays as described in Materials and Methods. Data are expressed as mean ± SEM (for A: n = 3 independent LPS-injection experiments; for B: n = 2 independent LPS-injection experiments). (C) Kinetics of LPS-induced MIP-1α (open circles) and MIP-1β (filled circles) appearance in murine macrophage cultures in vitro. Parallel cultures (0.5 × 106 thioglycollate-elicited peritoneal exudate macrophages) were incubated with 200 ng/ml LPS and the conditioned medium removed at specified intervals for ELISA analysis. Data are expressed as mean ± SEM (n = 3 independent experiments).