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The recent communication by Bishop and Gray (J. Biochem. 90, (1981)) on the streaming in the circular slit of stream cells is probably based on observations under conditions where no active streaming can be produced because of a substantial lack of F-actin. According to our experience, these conditions are similar to those obtained when actins are partly denaturated. Hence, for the sake of their criticism, detailed comparisons should be added of the stability of streaming, the distribution of streaming velocities as well as their sizes, acto-HMM ATPase activities under various streaming velocities and the amount of proteins fixed to the Millipore filter.