An assay has been developed to assess the dynamics of cell surface glycoproteins, in which neuraminidase digestion of intact cells is used to determine the fate of cell surface molecules initially labelled via lactoperoxidase-catalyzed iodination. This approach has been used to demonstrate the constitutive endocytosis and recycling of the T3-T cell receptor (T CR) complex on the human T leukemic cell line HPB-MLT. Stable populations of both phosphorylated and nonphosphorylated forms of the T3 gamma peptide have been identified in these cells. Whereas the former are constitutively endocytosed, the latter appear to be excluded from this pathway. The results presented indicate that T3 gamma phosphorylation may control the endocytosis and recycling of the T3-TCR complex on this cell line.