A study was conducted on 51 epileptic patients divided into two groups. Group 1: 20 patients without anticonvulsant treatment for at least two years, and group II: 31 patients receiving phenytoin in a dose of 300 mg/24 hr for at least 4 months. A group of normal and blood donor subjects were used as control. The serum concentrations of IgA and IgM were significantly decreased in the phenytoin treated patients in comparison with control. Non-treated epileptics showed a significant decrease of IgA level in comparison with control. It was suggested that phenytoin treatment suppresses the normal function of the humoral immune response and that epilepsy may be a contributing factor.