Alpha adrenergic stimulating drugs, metaraminol, norepinephrine, phenylephrine, was found to increase, in vitro, immunological phagocytosis by mice peritoneal macrophages. This effect could be inhibited by dibenamine, a blocking agent. The stimulation by alpha adrenergic agents was similar to that caused by drugs that reduce the intracellular levels of cyclic adenosine monophosphate or by drugs that increase the levels of cyclic guanosine monophosphate.