Acidic ethanol extracts of human platelets induced non-neoplastic normal rat kidney fibroblasts to undergo anchorage-independent growth. Less than 100 ng/ml of the crude extract elicits 50% of the maximal biological response when assayed in the presence of epidermal growth factor (2.5 ng/ml). In the absence of epidermal growth factor, the potency of the extract decreased 1,000-fold. These results show that platelets contain a type beta transforming growth factor. The specific activity of the platelet extract is 100-fold greater than that of other non-neoplastic tissues. The growth factor was purified to homogeneity by sequential gel filtration on Bio-Gel P-60 columns, first in the absence and then in the presence of urea. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, this transforming growth factor-beta is a protein of 25,000 daltons. It is composed of two 12,500-dalton subunits held together by disulfide bonds. These results, as well as its amino acid composition and its lack of strong mitogenic activity, show that this protein is distinct from platelet-derived growth factor. When completely purified, transforming growth factor-beta elicits 50% of its maximal biological response at concentrations less than 5 x 10(-12) M.