Macrophage functional competence was studied in two congenic chicken lines 6.6-2 (B2B2) and 6.15-5 (B5B5) which are regressors and progressors, respectively, of Rous sarcoma-induced tumors. Sephadex-elicited abdominal exudate cells (AEC) were harvested from 4-week-old chickens to determine their total number, glass adherence potential, percentage of adherent macrophages and phagocytosis of antibody coated (ops) and uncoated (unops) sheep red blood cells (SRBC). Tumoricidal abilities of culture medium conditioned with lipopolysaccharide treated macrophages and of macrophages cocultured with target cells were assessed against 51Cr-labelled tumor cell targets. The congenic lines did not differ in total AEC or percent macrophages. However, AEC from B5B5 birds exhibited significantly lower (P < 0.05) glass-adherence potential than AEC from B2B2 birds exhibited significantly lower (P < 0.05) glass-adherence potential than AEC from B2B2 birds. The percentage of phagocytic macrophages did not differ between lines for unop-SRBC, whereas a higher percentage of B5B5 compared with B2B2 birds (P < 0.05) macrophages phagocytized ops-SRBC. Macrophages from B5B5 birds had significantly (P < 0.05) lower activity in both tumoricidal tests. These results imply that the tumor progression in B5B5 birds is associated with reduced activation of macrophages towards a tumoricidal pathway.