Approximately 30% to 40% of patients with acute leukemia receiving repeated pooled random-donor platelet transfusions develop anti-HLA alloantibodies. Over time, however, serum anti-HLA concentrations decrease in approximately 50% of these patients, despite continued exposure to platelet and/or red blood cell transfusions. Using an enzyme-linked immunosorbent assay to measure serum Igs, the present study demonstrates that the sera of 67% of 82 transfused patients exhibiting a decrease in anti-HLA contain antibodies (anti-idiotypes) that react with the variable (V) region of anti-HLA antibodies. Anti-HLA binding to platelet membranes could be inhibited by these serum antibodies in 36% of the patients, indicating they had paratope-related reactivity. Protein G sepharose absorption showed that the anti-HLA V region-reactive antibodies were IgG. Of the 43 patients who had a decrease in anti-HLA levels, that were 16 whose anti-HLA decreased to undetectable levels; 7 (44%) developed anti-idiotypic antibodies that could specifically inhibit their own previously anti-HLA-positive serum. In contrast, antibodies with reactivity to the V region of anti-HLA antibodies (anti-idiotypes) were not demonstrable in patients who developed anti-HLA that did not decrease or disappear. The findings suggest that the development of anti-HLA V region-reactive antibodies (anti-idiotypic antibodies) correlates with a decrease in anti-HLA antibody formation in patients multiply transfused with platelet concentrates. The observations indicate that anti-idiotypic antibodies may downregulate alloimmune responses in patients undergoing repeated allostimulation during platelet transfusion therapy.