Protein A of Staphylococcus aureux is a marker probe for the Fe part of IgG-1, −2 and −4 globulins and therefore of potential use in immunological work. A one-step method for the rapid purification of protein A is described Staphylococcus aureus strains were grown in broth at 37deg;C and the protein A containing supernatants were mixed with immunosorbent containing protein A-reactive normal human gammaglobulin. The excess broth was then washed away and protein A finally eluted off with 2 to 3 M KSCN. Using normal human gammaglobulin coupled to Sepharose 4 B the protein A preparations obtained were pure as judged by disc electrophoresis. Some previously described gel precipitation phenomena related to immunoglobulin reactivity with protein A, all occurred in a similar way using preparations of protein A produced by two different strains of Staphylococcus aureus.