We have studied the production of transforming growth factor (TGF) by several human tumor cell lines and their interactions with exogenously added epidermal growth factor (EGF) and insulin. TGF-like activities were present in all the conditioned media tested. The clonogenic capacity of the tumor cell lines had no correlation with the TGF-like activity production. EGF and insulin had a promoting colony-forming activity on tumor cells but this effect was not additive. Moreover, an inverse statistically significant correlation (-0.817, p less than 0.05) was found between the response to exogenous EGF and the EGF or TGF-alpha production by tumor cell lines. The EGF receptor (EGF-R) was not detected in any of the melanomas studied, nor in breast adenocarcinoma cell lines which were producers of EGF or TGF-alpha.