In the present study we have examined the proteins involved in the insulin signaling cascade during and after differentiation of human adipocyte precursor cells and their correlation with glucose uptake, The differentiation of human adipocytes was characterized by a two- to threefold stimulation of glucose transport in response to insulin and a marked increase protein expression for the insulin receptor, IRS-1, GLUT-4, PI 3-kinase, and PKB, with respect to undifferentiated cells, In contrast, there were small changes in the protein expression of IRS-2, and no changes in PHC zeta and MAP kinases, although basal MAP kinase activity and GLUT-1 protein were reduced during differentiation. In conclusion, there are quantitative differences in the regulation of IRS-1 and other proteins during differentiation which may contribute to more efficient insulin signaling leading to glucose uptake in mature fat cells, Alterations in this pattern. may reflect or contribute to an insulin-resistant state,