Cell lines from SCLC were established with a success rate of 43% from different metastatic sites of treated and untreated patients. All 6 SCLC cell lines grew as floating cell aggregates without substrate adherence. The degree of aggregation ranged from very tight spheroids to very loose sheets and chains. This gross morphological property showed a striking correlation to the PDT, with short PDTs in loose growing cell lines and long PDTs in tight growing cell lines. Cell size and nuclear features, i.e., chromatin pattern and nucleolar prominence, also seemed to correlate with the PDT and gross morphology. All SCLC cell lines had dense core granules by electron microscopical examination. Several different serum-free and serum-supplemented growth media were tested for their feasibility in establishing and permanently growing SCLC. Serum-free SIT medium and SIT2.5 medium provided the best results in liquid culture. For semisolid SCLC cultivation, R 10 medium was superior to all other media tested. These cell lines are currently under intensive biochemical, molecular biological, and cytogenetical investigation in different laboratories and thus provide a tool for studying the biology of lung cancer.