Change in peripheral blood WBC and differential cell count in response to oral glucocorticoids (steroids) was examined in 36 stable renal transplant patients receiving their usual steroid dose on a daily or alternate-day steroid schedule. Three hours following steroid therapy mean WBC count had increased significantly. Mean change in WBC count was +2,400 cells/cu mm with individual values ranging from -600 to +8,000/cu mm. No differences were observed between patients receiving daily or alternate-day regimens. Changes in WBC count were due almost entirely to an increase in segmented granulocytes and a decrease in lymphocytes. there was no correlation between dose of steroid and WBC responses. However, when retested, a given patient's WBC response to a given dose of steroid was reproducible. Differences between patients, with respect to WBC response to steroids, could not be explained by differences in azathioprine dose and was not related to initial WBC count hematocrit value, age, duration of transplant, or levels of serum creatinine, BUM, or serum phosphorus. Because of the clinical importance of the WBC count in the renal transplant recipient and the potential for large and unpredictable changes in WBC count in response to steroids, WBC and differential cell count should be obtained before the morning steroid dose.