45Ca uptake was studied in isolated frog retinal pigment epithelial cells in response to the phagocytic stimuli, zymosan.45ca uptake was strongly stimulated immediately in the presence of zymosan particles. Calcium uptake was proportional to the zymosan concentrations. After 60 min in the presence of zymosan acid phosphatase andβ-glucuronidase activities showed a 25% and 50% increase, respectively. Rod outer segments induced a similar increase of these enzyme activities. The zymosan-induced lysosomal enzyme activities was inhibited by cytochalasinβand ruthenium red. The ionophore A23187 produced a remarkable increase in45ca uptake but did not affect the lysosomal enzyme activities. These results suggest that _in vitro RPE cells are able to respond to zymosan as phagocytosable stimuli and that calcium mediate that response.