Skin fibroblasts of patients with Duchenne muscular dystrophy were analysed for HLA tissue antigens. For the examination of the antigens two modified methods were used: the microdroplet cytotoxicity assay on fibroblast suspension and the cytotoxicity assay on fibroblast monolayer cultures using fluorochromasia. The HLA determination on the fibroblast suspensions of the patients showed a variety of unspecific HLA expressions which were not found in fibroblasts of healthy controls. These reactions were not apparent in the test in monolayer cultures. The exact cause has not been elucidated yet, but it appears to have a strong relation to some membrane abnormality in cells of DMD patients. The reproducible lack of unspecific reactions on monolayer cultures lead us to propose that these expressions arise from artificial injuries to membranes of these patient cells during trypsination or centrifugation. The validity of this test has been confirmed by a blind study.