Immunoglubulin concentrations were determined by radial immunodiffusion in sera from 15 epileptic patients before and during phenytoin therapy. Three reaction patterns were recorded: Two patients developed IgA deficiency (less than 0.05 mg/ml) during the first 3-4 months of treatment. Both patients also had a decrease in serum IgG and IgM, but no significant fall or increase in serum IgE. The IgA deficiency state was apparently reversible, since normalization of serum levels occurred after withdrawal of phenytoin. Five patients developed a 35-80 per cent reduction in serum IgA. In these patients, the decline in serum levels of IgG and IgM was inconsistent. Eight patients showed no significant fluctuations in serum immunoglobulins during phenytoin treatment. When a fall in serum IgA occurred, it did not correspond to a fall in serum or in red cell folate. Mean serum IgG was lower (9.37 mg/ml) in epileptic patients who had taken phenytoin for less than 1 year and had a low IgA, than in patients who had taken phenytoin for 10 years or more (11.50 mg/ml).