In vivo labeling with [35S]cysteine has identified three transmembrane forms of the rat hepatic polymeric IgA receptor: (i) a 105-kDa core glycosylated precursors; (ii) a terminally glycosylated 116-kDa intermediate; and (iii) a mature 120-kDa form. In the current study we show that the 120-kDa form is phosphorylated. After in vivo labeling with [32P] orthophosphate, all receptor forms were immunoprecipitated from hepatic total microsomes (TM) (with an antireceptor antiserum), separated by NaDodSO4/PAGE, and detected by autoradiography. The 120-kDa form was selectively phosphorylated, whereas the 116- and 105-kDa forms incorporated no detectable 32P. To determine the topology of the phosphorylation sites, hepatic TM isolated from rats labeled in vivo with either [35S] cysteine or [32P]orthophosphate were treated with trypsin. TM were solubilized and receptors were immunoprecipitated from lysates. With increasing trypsin concentrations, the [35S]cysteine-labeled receptor triplet was degraded to a trypsin-resistant doublet of ≃95 and 85 kDa, indicating that ≃20 kDa was removed from the receptor endodomain by trypsin. The same treatment removed all detectable 32P from labeled receptors. Furthermore, no 32P was detected in the 80-kDa biliary form of the receptor. Serine was identified as the only phosphorylated residue in acid hydrolysates of 32P-labeled immunoprecipitated receptor. These findings indicate that (i) the 120-kDa form is the only phosphorylated species of the receptor; and (ii) the phosphorylated residues are serine(s) located in the endodomain of the protein.