A mouse tumor line P388D1 passaged in tissue culture for many years has been characterized morphologically and functionally as a macrophage like cell. P388D1 cells phagocytize latex particles and firmly adhere to glass and plastics. In addition they have been shown to carry cell-bound receptors for immunoglobulin (Fc) and complement (C3). They fail to stain with fluorescent anti-mouse Ig or heterologous anti-mouse. Functionally, these cells exhibited high effector activity in an antibody-dependent, cell-mediated cytotoxic system. The accessibility of well characterized and pure macrophage-like cell lines, such as P388D1, will facilitate studies where cell purity is essential.