Serum IgA, IgG and IgM concentrations were determined in 30 patients with partial epilepsy before and during carbamazepine therapy. None of them had received anti-epileptic drugs prior to the study. The IgA and IgM concentrations decreased significantly during the 1st month of treatment (P less than 0.0001). No further decrease was observed during the subsequent 3 months. 15 patients were followed for 8-30 months. The IgA concentrations remained low, while the IgM concentrations tended to increase. However, the IgM concentrations did not reach the levels which were found before carbamazepine was started. Carbamazepine had no influence on the mean IgG serum concentration. There was no relationship between the IgA and IgM concentrations in serum and the serum concentrations of carbamazepine.