Epstein-Barr virus (EBV) transformed B-cells from patients with multiple sclerosis (MS) and healthy controls were analysed for reactivity with the HLA-DQ-specific human-human hybridoma Ab TrB12 (anti-DQw6 + DQw8 + DQw9) by indirect immunofluorescence (IIF). Positive results were obtained with 34 out of 35 MS patients (97.1%) and 79 out of 106 controls (74.5%) (p less than 0.005, RR = 11.6). Thus, DQ molecules that express the TrB12 epitope may contribute to the susceptibility to develop MS.