The human tumor cell lines, MM-96, FME, HCT-8, HT-29, MCF-7 and T-47D, in culture produced a factor or factors able to stimulate glycosaminoglycan (GAG) synthesis in human skin fibroblasts (HF). Conditioned growth media from the melanoma MM-96 and the colon carcinoma HT-29 produced a 10- and 8-fold stimulation of HF GAG synthesis, respectively, with an even larger stimulation of hyaluronic acid. Conditioned media from the melanoma FME and the breast carcinomas MCF-7 and T-47D stimulated GAG synthesis 2-fold, whereas media from the colon carcinoma HCT-8 gave a variable response often with no effect on GAG levels. Conditioned media from HF cultures had no effect on tumor cell GAG synthesis. Coculture of tumor cells and HF also resulted in increased GAG synthesis, and the degree of stimulation was similar to that with the conditioned media. Tumor cell-conditioned media were also effective in stimulating GAG synthesis by porcine smooth muscle cells and by chick embryo fibroblasts in culture, although the increase in GAG synthesis was much less than with HF cultures. These findings support the concept that the stromal desmoplasia characteristic of many growing and invasive tumors in vivo arises by tumor cell modulation of GAG synthesis by surrounding normal connective tissue cells.