This paper describes the morphology, phagocytic capacity, and response to lipopolysaccharide (LPS) of peritoneal macrophages (Mf) from Japanese quail, following induction by thioglycollate.+) Analysis by transmission and scanning electron microscopy : Similar to chicken Mf, non-activated (.῎) quail Mf had many gathering processes on the cell surface. Activated (-1῎) quail Mf exhibited a few long, extended pseudopodia. The cell nuclei of both non-activated and activated quail Mf di#ered from chicken cell nuclei in that they contained one large and several smaller heterochromatic bodies, which were attached to the nuclear membrane and other quail somatic cells.,) Phagocytic activity for sheep red blood cells (SRBC) : The phagocytic activity for SRBC of Mf from non-immunized quail (niMf) was significantly less than that of Mf from SRBC-immunized quail (imMf) (p῍*.*/). However, the phagocytic activity of niMf significantly enhanced by treatment of the SRBC with anti-SRBC quail serum (p῍*.*/). In contrast, the phagocytic activity of imMf did not change following treatment of the SRBC with anti-serum. These results suggest that the phagocytic activity of quail Mf for SRBC is enhanced by opsonization, but that levels of the opsonin receptor on the cell surface of Mf might be decreased by immunization with SRBC.-) Stimulation with LPS : The activity of LPS-stimulated Mf (MTT assay) was greatest *./ hours after cultivation began. Following LPS stimulation, quail Mf produced molecules of .../ kDa and 1-.-kDa, probably interleukin-0 (IL-0) and carrier protein complexes.