Serum-free medium conditioned for 72 h by a human undifferentiated adenocarcinoma of lung, Cal u 6, stimulated the colony formation of normal human bronchial epithelial cells, newly cultured cells from human solid lung tumors, and established human lung tumor cell lines, including Cal u 6 cells themselves. The activity was concentration dependent and was stable to acid. Growth factors in Cal u 6 conditioned medium supported culture of solid lung tumors; primary cell cultures were obtained from 9 out of 10 solid lung tumors of non-small cell origin and from 1 small cell tumor using Cal u 6 conditioned medium. In addition, 2 cell lines have been established to date from these primary cultures. Gel filtration of concentrated Cal u 6 conditioned medium on Biogel P-10 separated the growth promoting activity into 4 regions. The results suggest that autocrine factors may be important in non-small cell lung tumor cell growth and that differences in response to EGF and TGF varies as may provide the basis for selective culturing of normal and neoplastic lung epithelial cells.