Abstract Purified rat serosal mast cells were labeled either with [ 32P]orthophosphate or [ 35S]methionine and their receptors for immunoglobulin E were isolated by repetitive affinity chromatography. In 35S-labeled receptor preparations SDS polyacrylamide gels revealed a broad receptor band, M r 45,000 to 53,000, and two other bands, M r 30,000 and 16,000, which apparently represent receptor-associated proteins. Only the receptor band was labeled by 32P. Phosphorylation of receptor was markedly stimulated by the divalent cation ionophore A23187, a known stimulator of histamine release, with changes occuring as early as 15 seconds. This early increase in receptor phosphorylation may be involved in the control of mediator secretion.