Previous work has shown that bpV (phen) induces potent insulin-mimicking effects in the rat; selectively activates the endosomal (EN) insulin receptor kinase (IRK) in liver; and markedly abolishes endosomal IRK-associated PTP activity (IRK-aPTP) while reducing that of total ENs by 30%. Here we examined the relatively selective effect of bpV (phen) on endosomal PTP activities for the purpose of defining IRK-aPTP(s). Using an in-gel PTP assay we detected multiple (20) species of endosomal PTP (30 to > 220 kDa) with 5 being markedly inhibited following in vivo bpV(phen) administration. Using a combination of Mono Q anionic exchange chromatography and immunoblotting we demonstrated that LAR, PTP-, and PTP-1B were present in endosomal subfractions not significantly inhibited by bpV (phen). PTP-1B activity was assayed in immunoprecipitates from hepatic ENs of control and bpV(phen)-treated rats and was found to be inhibited by 30% following bpV (phen) treatment. To clarify further the role of PTP-1B in dephosphorylating IRK we prepared hepatic ENs from wild-type and PTP-1B null mice and found that the phosphotyrosine content of IRK was similar in these two types of ENs, and that IRK dephosphorylation was not affected in ENs from PTP-1B null mice compared with that in ENs from wild-type mice. These data suggest that LAR, PTP- , and PTP-1B are not candidates for the IRK-associated PTP in hepatic ENs, and that IRK dephosphorylation in ENs may result from the concerted action of several PTPs.