Antibody titers to diphtheria, pertussis, tetanus, and poliomyelitis (types I to III) were measured in previously vaccinated children with acute lymphocytic leukemia in remission after cessation of therapy. The response to revaccination one year after therapy was stopped was also studied. The patients' antibody titers were compared with those of healthy children, matched for age and sex. Two groups of patients were studied: one group (group A, N = 30) was given two drugs (6-mercaptopurine, methotrexate); the other group (group B, N = 19) was given three drugs (6-mercaptopurine, methotrexate, and cyclophosphamide) for maintenance treatment. In general, the patients' antibody titers were lower than those of healthy children, but in most patients they were still at levels considered to be protective. No significant differences in antibody levels between the two patient groups were found. A spontaneous rise in antibody titers in the first year after termination of therapy was not observed. After revaccination the rise in antibody titers was correlated with preexisting antibody titers in the same way in patients as in healthy children, and the antibody titers in patients and in healthy control subjects were on roughly the same level.