Thirty-nine children with ALL who had completed three years of chemotherapy were randomized to receive oral BCG for immunotherapy or no treatment as controls. There was not a significant difference between the two groups in the relapse rate. Among the immune parameters, only in vitro blastogenic responses to PHA and PPD rose significantly in the BCG group compared with the controls. Skin testing also revealed evidence of tuberculin sensitization. The group as a whole was studied for the kinetic recovery of immune functions after the cessation of chemotherapy, which revealed a dissociation in both cellular and humoral systems. At three weeks after therapy, only peripheral blood lymphocyte count, non T-cells, and serum IgM showed a significant abnormality. There was a rise in these parameters in the subsequent weeks, and the non-T-cell count reached normal levels sooner than the other two parameters. Children who were less than 5 years of age at the time of diagnosis showed a lesser degree of immunosuppression after long-term chemotherapy compared with those who were greater than or equal to 5 years of age. The analysis of the data indicated a relationship between the low serum immunoglobulins (IgG, IgA) and disease status.