Previous studies have indicated that factor VIII concentrates have a direct inhibitory effect on the proliferation of peripheral blood mononuclear cells (PBMC) obtained from normal volunteers. We have studied the effects of an intermediate and high purity factor VIII concentrate on haemophiliac PBMC. Intermediate purity factor VIII concentrate inhibited PBMC proliferation, the high purity factor VIII concentrate had no effect. The high purity factor VIII concentrate increased production of the lymphokine interleukin 2, suggesting the presence of a clone of T cells sensitized to factor VIII.