The effect of cholera exotoxin and aminophylline on Fc receptors in a murine lymphoid-cell line and in rabbit pulmonary alveolar macrophages has been investigated. Although both agents elevated intracellular cyclic AMP levels in macrophages and lymphoid cells, the effects on Fc receptor expression were distinct. Cholera toxin at 10 microng/ml reversibly inhibited Fc-receptor activity in the murine lymphoid cell line. In contrast, cholera toxin at 10 microng/ml or 0-01 microng/ml was ineffective in altering pulmonary alveolar macrophage receptor expression. Fc receptor activity on the macrophage was reduced by 20-30 per cent following incubation with aminophylline, (10(-3)M) from 0-6 h. There was no direct correlation between Fc-receptor activity and cyclic AMP levels in the cells studied. The differential susceptibility of these lymphoid and phagocytic cell populations to cholera toxin and also toward aminophylline suggests that there may be fundamental differences in topography on the membrane surface, or in the intracellular regulation of Fc receptors between lymphoid and phagocytic cells.