Antiidiotypic antibody (AIA) was raised in mice by immunization with MOPC 315 immunoglobulin A emulsified in Freund's adjuvant (FA). The antibody content of mouse serum was assessed by (a) its ability to inhibit rosetting of 2,4,6-trinitro-phenyl-sheep red blood cells around MOPC 315 myeloma cells, and (b) by a solid phase antigen-binding plate assay based on reactivity with 125I-Protein A and inhibition in the presence of dinitrophenyl aminocaproic acid. FA was necessary for the production of AIA to MOPC 315 immunoglobulin A. Some of the AIA-containing mouse sera were cytotoxic for MOPC 315 cells in the presence of guinea pig complement. However, cytotoxicity was not correlated with amount of AIA, as assessed by inhibition of rosette formation, nor was it specific for myeloma cells bearing the MOPC 315 idiotype. Furthermore, cytotoxicity could also be generated by immunization of mice with complete Freund's adjuvant, incomplete Freund's adjuvant, or the muramyl dipeptide portion of mycobacteria, all in the absence of MOPC 315 immunoglobulin A. Therefore, the complement-dependent cytotoxic antibodies in the AIA-containing antisera, which belonged to the immunoglobulin G and M classes, were likely directed against some component of FA. Myeloma cells which were not killed by anti-FA antiserum, as assessed by dye exclusion, were inhibited in their ability to secrete immunoglobulin and to form clones in agar.