Transforming growth factors (TGF) are polypeptides that stimulate anchorage-independent growth of various nontransformed cells in vitro. Transforming growth factors have been found in tumor extracts and in the urine of cancer patients. The specific questions of our study were whether TGF activity can be detected in malignant effusions, how different assays for TGF correlate with each other, and how assays for TGF correlate with soft agar tumor colony formation as measured by the human tumor cloning assay (HTCA). The TGF activity was measured by a normal rat kidney transformation assay (NRKA), a '*%EGF radioreceptor assay (RRA), and a radioimmunoassay (RIA) for TGF-a. Cells from effusions were cytologically examined and plated in the HTCA. A total of 104 effusions from cancer patients and 17 effusions from non-cancer patients was tested. Transforming growth factor activity was detected in some specimens. Only the TGF-a RIA and the HTCA showed significant differences between cancer and noncancer patients. Immunoreactive TGF-a was measurable in some cases by RIA even when cytologic testing failed to detect malignant cells. Spearman correlations between assays indicated that RIA results correlate significantly with all other assays. It is concluded that TGF-a activity might be important for in vitro colony formation of human tumor cells.