Screened and heated clotting factor concentrates of intermediate purity have been used in Taiwanese haemophiliacs since the end of 1986. A significant improvement of CD4/CD8 ratio during the years 1987-1989 as compared with those during the years 1984-1986 was observed in haemophilia A patients [mean +/- SD (median), 1.191 +/- 0.495 (1.163) vs. 0.880 +/- 0.325 (0.838), P = 0.0020] who were seronegative for human immunodeficiency virus. Almost all patients received an increased amount of factor VIII concentrates and total plasma products since 1987. Multiple linear regression analysis for the association of CD4/CD8 ratio with changes in dosage of plasma products revealed that there was a significant positive association of CD4/CD8 ratio measured during 1987-1989 with dosage of factor VIII concentrate administered during 1984-1986 (P = 0.0230), which is an indicator for changes in viral load, but not with changes in dosage of plasma products, which are indicators for changes in plasma protein intake. Our data indicate that immunological abnormalities after replacement therapy observed in haemophiliacs are mainly attributed to virus infection through infusion of factor concentrates, not to allogeneic proteins existing in plasma products.