We tested the efficacy of passive serotherapy in the treatment of acute lymphoblastic leukemia in four patients who had relapsed while receiving standard chemotherapeutic agents. Each patient received multiple intravenous infusions of J-5 monoclonal antibody specific for common acute lymphoblastic leukemia antigen (CALLA). In the three patients with circulating leukemic cells, there was a rapid decrease in circulating blasts that began immediately after antibody infusion, but not all leukemic cells were cleared, and remaining cells appeared to be resistant to further serotherapy. Although J-5 antibody was also demonstrable on bone marrow lymphoblasts immediately after antibody infusion in one patient, there was no change in bone marrow cellularity or differential during serotherapy. Analysis of the cell surface phenotype of leukemic cells during serotherapy and in vitro studies with patient cells suggests that resistance to serotherapy was mediated in part by antigenic modulation of CALLA in response to J-5 antibody.