Two culture cell lines (C-Lu65, C-Lu99) were established from human giant cell carcinomas of the lung transplanted in athymic nude mice (BALB/c, nu/nu). During early passage in tissue culture, C-Lu65 grew as a loosely adherent monolayer with some piling-up and with floating cells. After 30 successive subcultures, C-Lu65 began to grow in suspended cell clusters, showing a faster growth rate. C-Lu65 was characterized by multinucleated giant cells with large abnormal nuclei and prominent nucleoli. C-Lu99 grew as adherent cells, and fewer multinucleated giant cells were observed. C-Lu65 and C-Lu99 showed some ultrastructural differences in cell surface and cytoplasmic features. Chromosomal analysis revealed numerical and structural abnormalities in both cell lines. Cell-free supernatants from both cell lines stimulated the colony formation of mouse bone marrow cells in vitro. In addition, mice bearing tumors induced by transplanting C-Lu65 and C-Lu99 showed remarkable leukocytosis without evidence of infection. These results suggest that these two cell lines release colony-stimulating factor both in vitro and in vivo.