Screening of 43 healthy Danish haemophiliacs revealed a significantly lower helper/suppressor (H/S) ratio than in controls. 8 of the haemophiliacs had an H/S ratio less than or equal to 1.0. A significant negative correlation occurred between the total lifetime factor VIII treatment and the H/S ratio. However, high-dose factor VIII treatment given to patients with antibodies against factor VIII was not associated with immunological abnormalities. Children had a significantly higher H/S ratio than the adult haemophiliacs. Patients exclusively treated with Danish cryoprecipitate during the last year had a significantly higher H/S ratio than patients receiving preparations from other sources. This difference might, however, be explained by lower age and lower total lifetime dose in the group receiving Danish preparations. Haemophiliacs treated with American preparations did not differ immunologically from those treated with preparations of other origin. Total serum IgG was increased in 23% of the patients. This parameter was negatively correlated with the H/S ratio. The possible relation of the observed immunological alterations among otherwise healthy haemophiliacs to the acquired immune deficiency syndrome warrants further attention.