Human I-cell leukemia virus I (HTLV-I) is a transforming retrovirus etiologically associated with adult T-cell leukemia/lymphoma and troPical spastic ParaParesis. The number of HILV-I seropositive blood donors in the US is becoming alarmingly high raising concerns about contamination of transfusable blood products. Development of a highly-sensitive and highlyspecific blood-screening assay for HTLV-I antibodies in serum has been hindered by limiting quantities of certain viral proteins, including the most immunogenic protein, envelope. The proposed Phase I study outlines a strategy for expressing selected portions, as well as the entire envelope protein using synthetic DNA coding sequences. It is anticipated that this approach will allow these polypeptides to be expressed at high levels in E.coli. The expressed polypeptides will be analyzed biochemically and evaluated for their ability to detect antibodies to HTL7-I in human sera. In Phase II, promising candidate envelope polypeptides will be used to configure an enzyme-linked immunoassay (EIA) to detect antibodies to HTLV-I in human sera. The EIA will be evaluated for sensitivity and specificity using sera from a large number of normal and infected individuals. It is envisaged that a complementary set of recombinant-derived HTL7-I proteins will be configured into a high- quality blood-screening assay for exposure to this virus.