BALB/c mice immunized with phosphorylcholine (PC) produce antibodies which are predominantly of the T15 idiotype. Monoclonal anti-T15 antibodies have been generated in a number of laboratories by allogenic or syngenic immunization. Most of these mouse monoclonal antibodies react with idiotopes that are in or near the PC-binding site and require the presence of both T15 heavy and T15 light chain variable regions. By immunizing rats with T15 immunoglobulins, we have obtained monoclonal antibodies that recognize idiotopes that are not near the antigen-binding site. Four of these rat anti-T15 monoclonal antibodies react with free T15 heavy chains and with T15 heavy chains associated with irrelevant light chains while four other rat monoclonal antibodies require the presence of both T15 heavy and T15 light chains. This battery of rat anti-T15 monoclonal antibodies is useful in searching for heterogeneity within the T15 antibodies and in following the expression of the T15 heavy chain variable region in different strains of mice.