1. Filters comprising multiple layers of rabbit renal tubular basement membrane were constructed with conventional pressure filtration chambers. The effects of concentration-polarization on the behaviour of these filters was assessed by studying the filtration of proteins and of serum under turbulent (stirred) and unstirred conditions. 2. With stirring bovine serum albumin was effectively rejected by the filter barriers (sigma = 0.95) but rejection was diminished (sigma = 0.18) without stirring. The hydraulic permeability of the filters also fell without stirring. 3. In the presence of horse immunoglobulin G, a wholly rejected protein, the rejection of cytochrome c was increased and hydraulic flux was reduced. 4. Filtration studies of serum showed that serum protein was effectively rejected with stirring (sigma greater than 0.999) but rejection diminished when stirring ceased (sigma = 0.98). Albumin was the only protein detected in the filtrate with stirring but alpha- and beta- globulins appeared when stirring ceased. 5. These results show that concentration-polarization markedly affects the behaviour of these basement membrane filters in vitro, since without stirring a polarization layer of rejected protein is formed, which reduces hydraulic permeability and results in increased protein permeation through the filter.