As a model for the recognition of effete cells by their viable neighbours. BALB/c mouse thymocytes were coincubated with isologous peritoneal macrophages. The macrophages bound preferentially to thymocytes undergoing apoptosis, a mode of death induced in these cells by treatment with the glucocorticoid hormone methyl-prednisolone. Binding occurred in the absence of serum and was inhibited by N, N'-diacetyl chitobiose, N-acetyl glucosamine and, to a lesser extent, by N-acetyl galactosamine and D-galactose. L-fucose, D-mannose and N-acetyl neuraminic acid had no effect. The results suggest the presence of lectin-like molecules on the surface of the macrophage that recognize changes in the cell-surface carbohydrate of the apoptotic cell. The pattern of inhibition of binding by monosaccharides differs from that of previously described endogenous mammalian lectins.