It has been reported that keratocytes endocytose foreign particles both in vitro and in vivo, suggesting the active participation of keratocytes in corneal wound healing and host defense mechanism. To understand the cellular response and the fate of engulfed foreign particles, we investigated the changes in the lysosomal enzyme activities after keratocytes endocytosed latex beads or glutaraldehyde-fixed Staphylococcus aureus. Acid phosphatase activity in the cells endocytosing either latex beads or fixed bacteria increased significantly as compared with that in the control over the period of 12 hours. Changes in acid phosphatase activity depended on both the incubation periods and the amount of latex beads or bacteria added. The activity reached a plateau after 6 hours of incubation. When keratocytes endocytosed latex beads, the activity of N-acetyl-beta-D-glucosaminidase in the cells showed an increase similar to that of acid phosphatase. However, the N-acetyl-glucosaminidase activity in the cells endocytosing fixed bacteria did not increase significantly. These findings suggest that endocytosing keratocytes increase degrading enzyme activities in the cells to break down engulfed foreign particles. It is also suggested that the intracellular response is varied according to the character of the engulfed materials.