The effect of prednisolone on various immunological parameters was studied in patients with ulcerative colitis in complete remission. The study was designed as a double blind trial in which patients received either prednisolone or a dummy preparation and the following observations were made:(1) The mean lymphocyte count fell from 1738 cells per mm3 to 501 cells/mm3 4 hr after prednisolone was given but by 24 hr was significantly elevated to 2399 cells/mm3; thereafter it returned to normal levels. (2) Surface marker assays of lymphocytes forming spontaneous sheep cell (E), Fc (EA), and C3 (EAC) rosettes; and cells bearing surface immunoglobulin fluctuated in approximately the same pattern as the total lymphocyte count. (3) The mitotic response to a sub-maximal stimulating dose of phytohaemagglutinin (PHA) was significantly depressed 4 hr after steroid administration but returned to normal by 24 hr. (4) Spontaneous and PHA-induced lymphocyte mediated cytotoxicity fell significantly by four hours and remained depressed to the end of steroid administration. The PHA-induced cytotoxicity was still significantly depressed 7 days after steroid administration was stopped. (5) K-cell cytotoxicity did not follow the general pattern and was only slightly reduced at four hours being lowest after 24 hr and still depressed 7 days after cessation of steroid administration. (6) The number of plasma cells in the rectal lamina propria showed no significant change after one week of steroid administration. (7) No significant changes occurred in any of the above assays, in the control group. (8) Polymorphonuclear leucocyte counts rose sharply by 4 hr in the patients receiving prednisolone. There was also a smaller but significant rise in the control group. They remained elevated for 7 days in the group receiving prednisolone, and subsequently fell to normal levels. The control group had returned to initial levels by 24 hr.