Eight cultured cell lines were established from human small cell lung cancers. Every cell line showed the morphological and biochemical characteristics of small cell cancer. Changes in cell characteristics were observed in many of these cell lines when culture conditions were changed: "oat cell type" changed to "intermediate cell type" and vice versa when serum-free medium was changed to serum-supplemented medium; a deficiency of vitamin A in the medium caused a change to squamous cells and vice versa; and a tumor promoter (teleocidin B) enhanced the adherence of these cells to the surface of plastic culture dishes. These findings provide evidence that many small cell lung cancer cell lines can change their morphology with changes in the environment of the cells.