The phagocytic and microbicidal activities of alveolar and peritoneal macrophages were evaluated 4 and 24 h after a full-thickness scald burn of 26 to 28% body surface area in anesthetized rats. The contribution of serum factors to the macrophage functions was studied concurrently. The phagocytic activity of alveolar macrophages obtained 4 or 24 postburn was reduced approximately 65% below control values when they were incubated in media containing autologous serum and approximately 45% (below controls) when they were incubated in media containing normal (control) serum. A similar, although not as marked, decrease in the phagocytic activity of peritoneal macrophages was also demonstrated. Serum obtained from rats 4 or 24 h postburn had a significant suppressive effect on the phagocytic activity of control alveolar, but not peritoneal, macrophages. The intracellular microbicidal activity of peritoneal macrophages obtained at the postburn intervals and incubated in media containing either autologous or control serum was unaltered from control values. However, alveolar macrophages obtained 24 h, but not 4 h, postburn had a significant (approximately 80%) increase, above control values, in their microbicidal activity. Serum obtained 24 h, but not 4 h, postburn stimulated control alveolar macrophage killing ability. These data indicate that thermal injury induces a defect in the phagocytic activity rather than the microbicidal activity of macrophages. This phagocytic alteration is mediated, in part, by serum.