Peripheral lymphocyte counts, T-cell and B-cell populations, DNCB skin reactions, Mantoux reactions, mitogenic transformation using PHA, and determination of serum phenytoin concentration were carried out in 45 children with severe handicaps. Twenty two of these patients were receiving phenytoin treatment. When compared with control subjects, the skin reactions were significantly depressed in the handicapped children, amongst whom those treated with phenytoin had more impaired reactions than those who were untreated. No significant relationship between the results of the skin reactions and the other parameters was found.