Natural thymocytotoxic antibody (NTA) which occurs in high titer in New Zealand Black (NZB) mice was compared with AKR anti-θ (C3H) antiserum for ability to induce redistribution of C57BL/6 thymocyte surface antigen. NTA reacted with both thymocytes and brain cells bearing either the θ-AKR or θ-C3H alloantigens and was found to be an IgM antibody. Anti-θ (C3H) was purified to obtain the IgG fraction which was used in these studies. Neither NTA nor anti-θ antibody induced cap formation alone, however, both formed caps upon the addition of specific anti-immunoglobulin sera. Studies involving the sequential exposure of thymocytes to both antibodies demonstrated that θ and NTA reactive antigen co-migrate on the thymocyte surface. These studies suggest that NTA reacts with a determinant common to both the θ-AKR and θ-C3H alloantigens.