The effect of thymosin alpha 1 (THN alpha 1) and its NH2-terminal fragment (THN1-14) and COOH-terminal fragment (THN15-28) on non-small cell lung cancer (NSCLC) growth was evaluated. Using an anti-THN alpha 1 antibody, receptors were identified on NSCLC cells that were pretreated with 10(-6) M THN alpha 1. [3H]Arachidonic acid was readily taken up by NSCLC cells and THN alpha 1 significantly increased the rate of arachidonic acid release. THN1-15 slightly stimulated but THN15-28 and THN beta 4 did not alter arachidonic acid release from NCI-H1299 cells. In clonogenic growth assays in vitro, THN alpha 1 (10(-6) M) significantly decreased NSCLC colony number whereas THN1-14, THN15-28, and THN beta 4 were less potent. Using growth assays in vivo, THN alpha 1 (10 micrograms s.c./day) but not THN1-14, THN15-28, or THN beta 4 inhibited significantly NSCLC xenograft formation in nude mice. These data suggest that biologically active THN alpha 1 receptors are present on NSCLC cells and that native THN alpha 1 inhibits the growth of human NSCLC.