Human squamous carcinoma cells attach and spread on thrombospondin (TSP)-coated culture dishes but exhibit significant variability among individual cell lines in their degree of responsiveness. Using a highly responsive squamous carcinoma line and a cell line which is much less responsive (as well as a human melanoma cell line which does not respond at all in the adhesion assay), we have examined binding of exogenous radiolabeled TSP. The cells which were the most responsive to TSP in the adhesion assay bound the greatest amount of radiolabeled ligand. Binding was time- and dose-dependent, saturable, inhibitable with excess unlabeled TSP, reversible, and specific. The less-responsive squamous carcinoma cells bound only 25-30% of the amount of TSP bound by the highly responsive cells while the nonresponsive melanoma cells bound less than 10% of the amount bound by the highly responsive squamous carcinoma cells. Our previous studies (J. Varani et al. (1986) Exp. Cell Res. 167, 376) have shown that the highly responsive squamous carcinoma cells also synthesized the greatest amount of TSP as indicated by biosynthetic labeling studies. The less-responsive squamous carcinoma cells were intermediate in synthetic activity and no synthetic activity was seen with the melanoma cells. These findings suggest that the amount of ligand bound may determine the degree of biological responsiveness and that endogenously synthesized TSP may be the source of that ligand.