The relationship between the thymus and immunological competence of some rodents has been well documented (1), Both neonatal thymectomy and adult thymectomy followed by irradiation and bone marrow protection result in a diminished capacity to mount cell-mediated immune reactions. In addition some humoral antibody responses, for example to antigens such as heterologous erythrocytes (2), serum proteins (3-5), and haptenic determinants are impaired. In contrast other antigens, for example MS2 phage (6), Escherischia coli endotoxin (7), pneumococcal polysaccharide (8, 9), and polymerized flagellin (10) can stimulate relatively normal antibody production in thymectomized hosts, though frequently the range of antigen doses studied was limited. This paradox raises the problem of the role of thymusderived (T) 1 lymphocytes in those responses dependent on an intact thymus. Miller and Mitchell (11) have shown that T cells themselves do not secrete antibody in the classical sense but collaborate with thymus-independent (B) lymphocytes which then become antibodyforming cells (AFC). The importance of the helper function of T cells in antibody production in carrier-hapten systems has also been stressed by Mitchison (12), Benacerraf, and his colleagues (13,14) and Cheers et al. 2 However, this work does not clarify the difference between thymus-dependent and thymus-independent antibody responses.T h e purpose of the present communication is to examine this question in an in vitro mouse model using antigens of various physical forms. T h e flagellar