Epithelial cells obtained from normal human urothelium, a cell line derived from a papillary bladder carcinoma, and cells derived from an invasive carcinoma were grown in a serum-free fully defined medium. The interaction between these cell types and normal bladder stromal cells obtained by explant culture in serum were investigated in mixed cultures. These studies showed that normal urothelium was not responsive to the growth factors produced by cultured bladder fibroblasts and the cells did not grow at increased rates in association with living fibroblast layers. Cells derived from a papillary human bladder carcinoma cell line also did not associate well with fibroblast layers or show marked stimulation of growth by preformed layers of fibroblast cells. On the other hand, cells of the EJ carcinoma line, originally derived from a patient with highly invasive disease, easily infiltrated fibroblast layers and were strongly stimulated to grow by the presence of the stromal cells. This model system might therefore be used to determine key elements associated with malignant progression in human bladder carcinoma.