Various immunological parameters exploring both T- and B-cell functions were determined in a group of 74 symptom-free Italian haemophiliacs treated with commercial concentrate imported from the USA and were correlated with the presence in their serum of antibody to HTLV-III. There was a strong correlation between HTLV-III seropositivity and the amount of concentrate consumed. A significant correlation between HTLV-III seropositivity and T-cell alterations, such as T4/T8 ratio less than 1 and reduction in the absolute number of T4+ lymphocytes, or B-cell alterations such as hypergammaglobulinaemia and enhanced spontaneous IgG synthesis in vitro, was also observed.