Many human solid tumours, particularly lung tumours, contain different subpopulations, the presence of which can complicate diagnosis and treatment, yet few models exist for in vitro studies. We have found that DLKP, a human lung cell line established from a tumour histologically diagnosed as a ‘poorly differentiated squamous carcinoma’, contains 3 morphologically distinct populations. Three clones corresponding to these populations were established from the parental DLKP cells. Confirmation that the clones were derived from the parental population was obtained by DNA fingerprinting. The clones were designated M (mesenchymallike), I (intermediate) and SQ (squamous). On prolonged subculture, SQ and M can each interconvert with I, but SQ and M do not interconvert. We investigated the growth patterns of these isolated populations in monolayer culture, soft agar, spinner flasks and serum-free medium. In all but the latter assay, the parental DLKP cells grew faster than each of the clones, indicating some form of physiological co-operation between the clones. The growth of the clones themselves varied under the different assay conditions (DLKP-I showing greatest growth in monolayer, in serum-containing and serum-free media but, surprisingly, being unable to grow in soft agar, unlike the SQ and M clones). Addition of fibronectin permitted growth of DLKP-M and DLKP-SQ in serum-free medium at equivalent rates to those of DLKP and DLKP-I. In some cases, morphological adaptation to specific growth conditions was observed. Variation between the clones was also evident in their respective chromosome numbers (with the M clone being predominantly hyperdiploid and the other clones predominantly hypertetraploid) and in their ability to adhere to extracellular matrix proteins, with DLKP-M showing most rapid attachment. Electrical resistance studies revealed the absence of tight junctions from the parental line and clonal subpopulations. Extensive immunohistological studies showed that neither DLKP nor the clones express cytokeratin or any other epithelial marker examined, but neuroendocrine markers were present. Further analysis of these different clonal populations may help to reveal some of the mechanisms involved in lung tumour development and progression.