In The Netherlands, anti-human T-cell lymphotropic virus type I (HTLV-I) blood donor screening became mandatory in January 1993. Donations reactive in the enzymelinked immunosorbent assay (ELISA) screening test are confirmed with Western blot analysis (WB). Only WB-positive or indeterminate blood donors are notified and retested by polymerase chain reaction (PCR) . In accumulated data of the Dutch Blood Banks, only 2% of donors repeatedly positive in the anti-HTLV-I/II ELISA were WB-positive and all of these were also PCR-positive. However, 75% of the ELISA-positive blood donors have indeterminate WB results. In these cases the ELISA reactivities are nonspecific since all WB-indeterminate donors are negative in PCR . Current WB confirmation thus leads to the notification and often to the deferral of many WB-indeterminate blood donors as well as to high costs for PCR testing, illustrating the need for a more specific serological confirmatory assay. The aim of our study was to evaluate a newly developed anti-HTLV-I/II Recombinant Immunoblot Assay (RIBA) to confirm samples reactive to screening tests with a special emphasis on the ability of this test to resolve WB-indeterminate results in blood donors, without compromising the sensitivity.