Macrophages activated in vivo with Corynebacterium parvum and asbestos showed an increased sub-population of cells which capped spontaneously on incubation with fluoresceinated Concanavalin A compared to saline-induced control macrophages. This capping was unaffected by colchicine but was inhibited by cytochalasin B. The spontaneous capping of activated macrophages did not appear to be directly related to the total number of Concanavalin A receptors as measured by specific 125I-labelled Concanavalin A binding to activated and control macrophages. Capping also did not appear to be related to the position of Concanavalin A receptors as judged by trypsin accessibility of bound 125I-labelled Concanavalin A.