Mouse thymocytes were iodinated by the lactoperoxidase technique. On subsequent culture approximately 90% of the labeled surface material was released into the culture medium within 1 hr. Specific binding sites for phytohemagglutinin or concanavalin A could be detected in the medium by coprecipitation techniques with specific antisera to the mitogens. Treatment of surface-labeled cells with mitogen increased the release of surface material on culture, and indicated that some of the specific binding sites were released as complexes with the mitogen. These findings suggest a promising approach to the isolation and characterization of antigen-specific receptors from lymphocytes.