The role of microtubules and microfilaments in controlling the mobility of concanavalin A (ConA) receptors in the membrane of peritoneal macrophages was examined. Receptor mobility was assessed by counting the number of cells exhibiting cap formation following incubation with fluorescein-labelled ConA for 5 min at 37 °C. It was found that 1. 1. ConA binds to peritoneal macrophages in the form of multiple aggregates (patches), this being followed by rapid endocytosis. No spontaneous receptor capping is seen. 2. 2. Pretreatment of macrophages for 1 h at 37 °C with the microtubule disrupting drugs colchicine, vinblastine and podophyllotoxin induces ConA cap formation in 25-50% of cells. Cap formation is not induced by the glutathione-oxidizing agent diamide. 3. 3. Pretreatment of macrophages with cytochalasin B (CB), a drug interfering with microfilament function, does not facilitate capping. However, CB totally prevents the induction of cap formation by podophyllotoxin. 4. 4. Exposure of cells to the macrophage-specific lymphokine, migration inhibitory factor (MIF), partially prevents cap formation induced by microtubule disrupting drugs. It is concluded that in peritoneal macrophages, the mobility of membrane receptors for ConA is normally restricted by cytoplasmic microtubules and cap formation is only possible in the absence of microtubules. Cap formation also requires the presence of intact microfilaments. Drug-induced cap formation is antagonized by MIF, a mediator promoting microtubule assembly.