Cyclophosphamide treatment of chicks abrogated the induction of immunity against Marek's disease (MD) by the herpesvirus of turkeys, but immunity eventually developed if a sufficient interval was allowed to elapse between vaccination and challenge. Vaccinated, cyclophosphamide-treated chicks showed severe suppression of antibody and immunoglobulin production even when immunity to MD was present, suggesting that resistance was not mediated by humoral immunity. Bursa and thymus weights of chicks were markedly reduced 7 days after treatment with cyclophosphamide, but at 35 and 70 days the mean weight of the thymus, but not of the bursa, was considerably restored. The results suggest that cyclophosphamide interferes with vaccinal immunity by an effect on the thymus and cell-mediated immunity, and not by an effect on the bursa and humoral immunity.