A hemophilic patient treated with Factor VIII (F.VIII) concentrates developed a F.VIII inhibitory activity. The patient's plasma showed 2 anti-F.VIII/von Willebrand Factor (vWF) antibodies (Abs) of the IgG and IgA class respectively. The specific anti-F.VIII/vWF Abs were isolated and resulted to be IgG (20 micrograms/ml) and IgA (35 micrograms/ml). The plasma was subsequently fractionated by Protein A-Sepharose and the IgG and IgA containing fractions were separately analyzed for anti-F.VIII activity. Both fractions exhibited F.VIII inhibitory activity, but that corresponding to the IgA was lower than expected. Regarding the possible existence of a non-inhibitor Ab in the IgA containing sample, plasma was processed through an immunoadsorbent to which a F.VIII devoid vWF preparation had been previously coupled and further fractionated by Protein A-Sepharose, obtaining 2 IgA fractions. One of them exhibited F.VIII inhibitory activity while the other, which reacted with the F.VIII devoid vWF preparation, did not. Therefore, this latter one was considered as a true anti-vWF Ab. The IgG and IgA inhibitors were polyclonal and the IgG one was composed of the 4 IgG subclasses.