Although the cytoplasmic domain of the human FcγRIa α-chain lacks tyrosine-based phosphorylation motifs, it modulates receptor cycling and receptor-specific cytokine production. The cytoplasmic domain of FcyRIa is constitutively phosphorylated, and the inhibition of dephosphorylation with okadaic acid, an inhibitor of type 1 and type 2A protein serine/threonine phosphatase, inhibits both receptor-induced activation of the early tyrosine phosphorylation cascade and receptor-specific phagocytosis. To explore the basis for these effects of the cytoplasmic domain of FcγRIa, we developed a series of human FcγRIa molecular variants, expressed in the murine macrophage cell line P388D1, and demonstrate that serine phosphorylation of the cytoplasmic domain is an important regulatory mechanism. Truncation of the cytoplasmic domain and mutation of the cytoplasmic domain serine residues to alanine abolish the okadaic acid inhibition of phagocytic function. In contrast, the serine mutants did not recapitulate the selective effects of cytoplasmic domain truncation on cytokine production. These results demonstrate for the first time a direct functional role for serine phosphorylation in the α-chain of FcγRIa and suggest that the cytoplasmic domain of FcγRI regulates the different functional capacities of the FcγRIa-receptor complex.