Bromoacetyl cellulose was used to prepare a monospecific anti-IgA immunoabsorbent for the one-step isolation of IgA from small volumes of human serum. Two features were essential to this procedure. a) Anti-IgA globulin was freed of inert γ globulin by immunoabsorption and was used for coupling to bromoacetyl cellulose. The binding capacity of this immunoabsorbent was 15 mg IgA/g absorbent (using whole human serum). b) 3 M urea wash solution was used to minimize contamination of the eluted IgA by nonspecifically absorbed serum proteins. The IgA obtained by this method was 98.5% pure; it retained biologic activity and showed the electrophoretic pattern of IgA in serum.