The effects of human recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF) 1-1000 U/ml on the growth of human lung cancer cell lines have been studied in vitro. A panel of 10 small cell, 1 adenocarcinoma and 1 large cell lines was used with multidrug resistant sublines of 3 of the panel. The MTT assay was used to quantify cell numbers after 6-8 days' growth in the presence of GM-CSF. Neither growth inhibition nor stimulation of any of the cell lines in the presence of GM-CSF was observed. Any effects of this agent on residual tumour cells may not therefore present a problem during its clinical use to stimulate marrow regeneration after high-dose chemotherapy for lung cancer.