Opsin for immunological studies can be prepared free from S-antigen by affinity chromatography on Con A-Sepharose. This preparation,however,contains 2-6% Con A originating from the affinity medium. Con A as impurity disturbes lymphocyte transformation tests carried out with opsin as test antigen. We describe a method for the removal of Con A by immuno-adsorption to Protein A/anti-Con A/IgG in detergent. In addition, we have selected some detergents and detergent concentrations in which opsin can be purified and added to the lymphocyte culture medi urn.