The subpopulations of murine macrophages (M phi) that can be activated by endotoxin (LPS) and lymphokines to produce lymphocyte-activating factor, or interleukin 1 (IL-1), and to express cytostatic and cytolytic activities against tumor cells were characterized in 3 cell populations with immature and mature M phi in various proportions, namely, normal peritoneal cells, glycogen-induced peritoneal cells, and cultured bone marrow M phi. There is disagreement in the literature as to whether tumor cytotoxicity can be elicited from all M phi or from a subset of small, newly formed peroxidase-positive (p+) monocytes. Using the technique of cell size fractionation, which separates p+ and p- M phi to various extents, we have found that the magnitude of te response to LPS and lymphokines increases with cell size (and hence maturity or the degree of prior stimulation) and is not exclusive to p+ M phi. Our results are consistent with the concept that all M phi, irrespective of developmental stage, can exist in a continuum of activated states, depending on the nature of the activation signals.