Mice were treated for 7 weeks with doses of methyldopa somewhat exceeding those given to man, and mixed immunotoxic effects were observed. Daily subcutaneous injections of 5 mg (in 0.1 ml) methyldopa or saline (0.1 ml) did not generally alter body weights, except on day 19, when the methyldopa-treated mice weighed significantly less. During treatment, all mice were immunized twice with sheep red blood cells (SRBC) and bled four times. Anti-SRBC titers were not affected by methyldopa treatment, but leukocyte counts were dramatically decreased, and hematocrits to a lesser degree. Although in methyldopa-treated mice spleen and kidneys were increased in size, liver, lung, heart, and thymus size was not affected. These results are discussed in the context of other studies on the mode of action of methyldopa in eliciting an autoimmune anemia in man treated therapeutically with this drug.