1. The filtration properties of films of renal basement membrane were studied in vitro using pressure filtration chambers. 2. Retention of cytochrome c by the films was found to be dependent upon the filtration pressure indicating that it was transferred across the films by convective as well as diffusive flow. In contrast, serum albumin was transferred by diffusive movement only. 3. When solutions containing both cytochrome c and IgG were filtered it was found that increasing the filtration pressure reduced the flux of cytochrome c across the films. A similar phenomenon occurred when serum was filtered, less protein passed through the films at high filtration pressures. These phenomena are explained by concentration-polarisation effects. 4. The flux of cytochrome c through the films was found to decrease in a non-linear manner as the films thickness was increased. With thin films, the flux of cytochrome c increased in a non-linear manner as the concentration of the protein in the overstanding solution was increased. With thicker films the flux was linearly dependent on concentration. These findings are interpreted as supporting the view that movement of cytochrome c occurs, at least in part, by convective flow.