One-way mixed leucocyte reactions (MLR) were performed between blood leucocytes from control subjects or pairs of patients with one of the following diseases: multiple sclerosis, rheumatoid arthritis, psoriasis arthropatica, pelvospondylitis, SLE, or scleroderma. Cells from each subject were also simultaneously tested against cells from a healthy `standard' person. Impaired MLR were found in all these patient categories compared with a control group of both healthy individuals and patients with unrelated diseases. The results of the patient–standard combinations indicated that the impaired MLR were not due to deficiencies in the patients' lymphocytes preventing stimulation of or response in MLR. One possibility is that the cause lies outside the lymphocytes, perhaps in an impaired macrophage function.