Retinoblastoma cells of the Y79 line were co-cultivated with human fibroblasts and bovine iris and retinal pigment epithelium in tissue culture. The Y79 cells, which characteristically grow as a suspension culture, were found to attach directly to the fibroblasts and pigment epithelium on the flask surface. Electron microscopic examination of the fibroblast-retinoblastoma co-cultures revealed numerous pinocytotic vesicles lining the fibroblast cell borders that were in contact with the tumor cells. The retinoblastoma cells contained increased numbers of ribosomes, endoplasmic reticulum, and-mitochondria. Fibroblastic processes appeared to wrap around and engulf tumor cells. In both the iris and retinal pigment epithelium co-cultures with retinoblastoma cells, there were increased numbers of mitochondria in the tumor cells in areas adjacent to pigment epithelium but no pinocytotic vesicles were seen. The pigment epithelium attached to Y79 cells showed fewer processes than did the fibroblasts in co-culture. In summary, both fibroblast and pigment epithelium functioned as an effective carrier cell layer for retinoblastoma cells. In addition, we believe that the fibroblast layer removed substances secreted by the tumor cells via pinocytotic vesicles.