The neu proto-oncogene product, p185(neu) (HER2, c-ErbB-2), encodes a cell-surface tyrosine kinase receptor with high oncogenic potential, which correlates with increased tyrosine kinase activity and a rapid receptor internalization rate. To investigate the interactions and signal(s) leading to the endocytosis of Neu receptors, we employed lateral mobility and internalization studies. Fluorescence photobleaching recovery measurements revealed that activation of Neu receptors (induced by mutation or by agonistic antibodies) markedly reduced their mobile fractions. To elucidate the signals involved, other mutants, all carrying a constitutively dimerizing oncogenic mutation, were analyzed. A kinase-negative mutant and a mutant lacking all cytoplasmic tyrosine phosphorylation consensus sequences exhibited high mobile fractions, similar to nonactivated Neu. Retention of a single tyrosine autophosphorylation site (Tyr-1253) out of the five known such sites was sufficient to immobilize a large fraction of the receptor. For all mutants, internalization correlated with receptor immobilization and was blocked by treatments that interfere with coated pit structure, indicating that the immobilization is due to interactions with coated pits. This was supported by the coimmunoprecipitation of α-adaptin only with the constitutively activated Neu mutants. We conclude that activated Neu receptors become stably associated with coated pits via plasma membrane adaptor complexes (AP-2). Efficient Neu receptor endocytosis requires activation, a functional kinase domain, and at least one tyrosine autophosphorylation site.