We examined the proliferative responses of T cells of patients with systemic lupus erythematosus (SLE), their first-degree relatives, and healthy donors, to a human monoclonal antibody that bears a common anti-DNA idiotype, 16/6 Id, and to a murine, 16/6 Id-specific, monoclonal antibody. Both 16/6 Id+ and 16/6 Id-specific antibodies were previously shown to be involved in the induction of experimental SLE in mice. Here we show that T cells of fewer SLE patients, as compared with healthy donors, could proliferate to both antibodies. The difference between T cell responses of patients and controls to the 16/6 was found to be significant. The proliferative responses of T cells of first degree relatives of SLE patients to the anti-16/6 Id were found to be significantly lower compared with the responses detected in healthy donors and in SLE patients. The responses of T cells of SLE relatives to the 16/6 Id were found to be lower than those of healthy donors, but this difference was not significant. The present study suggests a possible involvement of T cells, and specifically of idiotype and anti-idiotype specific T cells, in SLE.