Since 2% of the cases of Acquired Immune Deficiency Syndrome (AIDS) have been attributed to transfusions of blood and blood products, licensed tests to detect antibody to the human T-lymphotropic virus type III (anti-HTLV-III) have been put into practice to reduce the risk of transfusion associated AIDS. Two commercial ELISA kits (Abbott and ENI) were used to test for anti-HTLV-III in 100 coded samples from individuals with AIDS, at high risk for AIDS, or with low risk for AIDS and in 1280 unlinked blood donor serums. From the 100 coded samples, both Abbott and ENI tests identified 51 of 52 coded samples with anti-HTLV-III which were confirmable with Western blot analysis. Initial testing of the donor serums by Abbott's test revealed 20 reactives, of which 5 were repeatably reactive; initial testing by ENI's test revealed 25 reactives, of which 14 were repeatably reactive. However, only 3 donor serums were repeatably reactive by both test kits, out of 17 repeatable reactive by either, and no ELISA positive samples were confirmed by Western blot or IFA. Before a blood donor is notified of "anti-HTLV-III reactivity", tests demonstrating this should be both reproducible and confirmable by at least one additional test.