Abstract: Samples e.g. transfusion blood, are assayed for antibodies to retroviruses, e.g. AIDS virus, using an insolubilized antigen comprising retrovirus antigens bound to globulin, the globulin itself being bound to an inert solid support; and an immunoglobulin which contains specific antibody to the retrovirus antigens and which is labelled with a revealing label, and the soluble phase is then separated from the insoluble phase and the quantity of revealing label associated with either the soluble or the insoluble phase determined.The sue of labelled antibody in competition with test sera for binding on the insolublized antigen permits better identification of antibody containing specimens. The retroviruses may be a human T-lymphotropic retrovirus HTLV-I, II or III or a new retrovirus isolate CBL-1 etiologically related to AIDS.