Sera from five patients with chronic aggressive hepatitis containing smooth muscle autoantibodies were tested by means of indirect immunofluorescence for their binding to isolated rabbit skeletal muscle myofibrils. In all cases, the immunofluorescent staining was sharply localized to I bands. After incubation of these sera with skeletal muscle troponin-torpomyosin complex, purified troponin or purified tropomyosin, no changes in immunofluorescent staining of myofibrils were noted. However, the staining was abolished after incubation of the sera with skeletal muscle actin. In double immunodiffusion experiments, a single precipitation line was obtained after diffusion of the sera against crude or purified actin. It is concluded that, at least for the sera examined, smooth muscle autoantibodies are anti-actin autoantibodies. The high titre of such autoantibodies and their availability in clinical immunology laboratories make them a useful tool to study actin distribution in muscular and non-muscular cells.