In order to address the question of the influence of a primarily chemoresistant tumor cell subpopulation on the progression of a heterogeneous tumor after cytotoxic therapy, in vitro established human small cell lung cancer cell lines of a 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-sensitive (592) and a resistant (NYH) tumor were used to produce mixed solid tumors in nude mice. Mixtures of 592/NYH (9:1 and 1:1) were inoculated s.c. After 3-4 weeks of tumor growth, the mice were stratified according to tumor size and randomized to treatment with BCNU 40 mg/kg i.p. (10% of lethal dose) or no treatment. Tumor growth curves were used to calculate the effect of the treatment, and changes in the relative proportions of 592 and NYH in the mixed tumors were monitored by flow cytometric DNA analysis by which the two cell lines were distinguishable due to differences in DNA content. A significant response was demonstrated in the 9:1 mixed tumors in which only 592 cells were detectable at the start of the treatment. The response was short and less pronounced compared with tumors containing only 592. In the regrowing tumors after treatment, only NYH was detected. In untreated 9:1 mixed control tumors, only 592 cells were detectable throughout the entire observation period. It is substantiated that the 592 cells were able to inhibit the growth of the NYH cells completely when grown together in 9:1 mixed tumors. This was not the case in the 1:1 mixed tumors. The 1:1 mixed tumors did not respond to BCNU, although 592 was eradicated. These results indicate that resistant and undetectable (dominated) subpopulations in heterogeneous tumors may be responsible for relapse and that the fractional size and the growth characteristics of the resistant subpopulation may determine the magnitude of the clinical response to cytotoxic treatment.