Tumor metastasis requires highly motile cells that can respond to appropriate stimuli. A2058 human melanoma cells were shown previously to secrete a highly potent autocrine motility factor (AMF) that stimulates chemokinetic movement. We have shown that the insulin polypeptides (IPS; insulin-like growth factors I and II [IGF-I, -111 and insulin) stimulated A2058 cell chemotaxis and chemokinesis. We now re- port that the IPS and AMF stimulate locomotion in other human malignant cell lines. Insulin (100 nn) induced motility of up to 50% of the magnitude of the AMF response in human carcinoma lines MDA-23 I (breast), T24 (bladder), and OVCAR3 (ovarian). The tumorigenic and metastatic 5R Haraotransfected rat embryo fibroblast cell line responded to insulin with both chemotaxis and chemokinesis and was 100% of that seen for AMF. The ED,, for IGF-I in the carcinoma cell lines was in the order of I nn, but the magnitude of the responses at this concentration was 40% of the AMF-stimulated response, with the exception of the A2058 cells, which were maximally stimulated at I nn. IGF-II induced maximal motility of 75 to 130% of the AMF-stimulated response in the carcinoma lines with ED, , of <I0 nn. IGF-ll-stimulated motility in the carcinoma lines was predominantly chemotactic by modified checkerboard analysis. Cell pretreatment with pertussis toxin inhibited 9&100% of AMF-induced motility, whereas migration to the IPS was not pertussis toxinsusceptible. In growth studies, IGF-I induced mitogenesis up to 140% of basal media control growth. In general, maximal growth stimulation was seen at 100 nn IGF-I, and optimal migration was seen at 10 nn IGF-I. The IGFs are secreted by normal stroma in a number of organs that are common sites for primary and metastatic disease. Therefore, we suggest that IPS may be important homing and mitogenic signals for tumor cells in the process of invasion and metastasis and that the differential motility stimulation and respective mechanisms of action by these physiologically important agents may underlie the diversity of the metastatic process.