Methods and reagents are provided for determining endocytosis, using a cell expressing an externally polypeptide labeled cell membrane receptor, and as a reagent an antibody to said label conjugated to a fragment of an enzyme fragment complementation pair. Compounds are tested for their effect on endocytosis by complexing the reagent with said cells, adding the test compound at any time in relation to the complexing, allowing any endocytosis to occur, proteolytically degrading external enzyme fragment, adding protease inhibitor and the complementary member of the enzyme fragment complementation pair and substrate. The product provides a detectable signal related to the amount of endocytosis that occurred. The method is readily automated as all steps can occur in a single vessel without separations and washings.