Two distinct cell lines were obtained from a single heterotransplanted tumor which had originated from a primary focus of small cell carcinoma of the lung (SCCL). They were maintained separately from the beginning in culture media with and without fetal calf serum supplementation. Cells in the serum-free medium grew mostly floating in loose aggregates and showed poor cell cohesiveness, scanty cytoplasm and a few intracytoplasmic small dense-cored granules; all of these features are characteristics of oat cell type SCCL. On the other hand, cells in the serum-supplemented medium grew mostly floating in flatter and more closely associated clumps, were larger, and showed increased cell cohesiveness, occasional tubular structures, better developed organelles including dense-cored granules, and an increased number of cell attachments; these features are characteristics of intermediate cell type SCCL. The modal number of chromosomes differed from each other. Neuron-specific enolase (gamma enolase) and aromatic L-amino acid decarboxylase (ADC) activities in cell pellets were significantly higher in both lines than in control non-small cell lung cancer cell lines. The alpha/gamma type enolase ratio was lower, as was the ADC activity, in serum-free cultures than in serum-supplemented cultures. Interchange of the culture medium induced changes of the growth pattern and cell type from "oat cell type" to "intermediate cell type" and vice versa. The chromosomal number also partially changed. These findings suggest that cultured cells of SCCL alter their growth pattern and cell type depending on the culture conditions and that the selective growth of one cell type might then take place.