The number of leukocytes and the percentage of cells in various mononuclear subpopulations were determined in 22 patients with epilepsy. The lymphocyte response to PHA were examined in the patients and in selected controls. The patients were grouped according to phenytoin treatment at present or previously, and according to the presence or absence of serum IgA depression. Patients taking phenytoin demonstrated reduced leukocyte counts. The lowest counts were found in phenytoin-treated patients with markedly reduced IgA concentrations. The number of lymphocytes were reduced more than the total leukocyte count in the patients. No significant differences were observed between patients and controls regarding percentage of cells with membrane-bound immunoglobulins, active E-rosetting cells, AET-rosetting cells, EA-rosetting cells, EAC-rosetting cells, and cells with a capacity of phagocytosis. The mean response to PHA stimulation (PHA concentration 0.6 micrograms/ml) was 4634 counts per minute in the patients and 6132 counts per minute in the controls. However, the response to this stimulation varied considerably between individuals in both groups, and the difference between the 2 groups was not statistically significant.