Cultured human retinal pigment epithelial cells were exposed to an immunotoxin composed of a monoclonal antibody, 454A12, directed against transferrin receptors conjugated to a toxin, recombinant ricin A chain. Exposure of proliferating human retinal pigment epithelial cells to the immunotoxin (0.1 to 10,000 ng/mL) caused a statistically significant (P less than .0001) decrease in the number of cells. This inhibitory effect was induced after an exposure to the immunotoxin as short as 5 minutes and was maximal after 24 hours of exposure. The diminution in cell number was dose dependent over the range from 0.1 to 100 ng/mL. Monoclonal antibody alone, recombinant ricin A chain alone, or an irrelevant immunotoxin, MOP21C monoclonal antibody-recombinant ricin A, did not diminish the number of cells. There was a marked decrease in DNA synthesis measured by nuclear tritiated thymidine incorporation that accompanied the immunotoxin-mediated decrease in cell number. Viable cells remaining after exposure to the immunotoxin (0.1 to 10,000 ng/mL) were morphologically abnormal; typically the cells had elongated spindle-shaped processes and had lost their normal cuboidal appearance. In contrast, cell number was not decreased in confluent human retinal pigment epithelial cells after treatment with maximal doses of immunotoxin. Morphologic changes similar to those seen in proliferating cells were observed in confluent cells exposed to more than 100 ng/mL of immunotoxin. The effect of the immunotoxin was species specific because large doses of immunotoxin did not reduce the number of viable cells in proliferating or confluent pig retinal pigment epithelial cells or cause observable morphologic changes in this cell type. Our results indicate that the immunotoxin selectively inhibited proliferating retinal pigment epithelial cells by receptor-mediated internalization of the antitransferrin receptor monoclonal antibody-recombinant ricin A chain conjugate.