Insulin-like growth factor I (IGF-I) has been implicated to play a regulatory role in T cell development and in T cell function. We investigated the expression of type I IGF receptors on human peripheral T cells related to the maturation and activation stage using the type I IGF receptor-specific monoclonal antibody alpha IR3. It appeared that 87% of the CD4+CD45RA+ cells and 66% of the CD8+CD45RA+ cells were alpha IR3+, whereas only 37% of the CD4+CD45R0+ cells and 38% of the CD8+CD45R0+ cells bound alpha IR3. We also found that the fraction of alpha IR3+ cells within in vivo or in vitro activated (HLA-DR+) T cells is markedly lower than in nonactivated (HLA-DR-) cells. In vitro phytohemagglutinin-activated T cells and CD4+CD45R0+ cells activated with recall antigens also contained less alpha IR3+ cells (1-6%) than nonactivated cells (30-54%).