Convective ultrafiltration using commercially available artificial membranes with selective sieving properties is a widely used method of molecular separation. We had previously reported an ultrafilterable inhibitor of toad bladder sodium transport in whole canine plasma using this technique. In the present study, two variables which have large effects on ultrafiltration of the transport inhibitor were identified. A relationship between stirring speed during ultrafiltration and recovery of the inhibitor was found, such that detectable recovery of the inhibitor was shown only at high stirring speeds. These results interpreted in terms of the gel layer model of the concentration polarization phenomenon indicate a profound effect of concentration polarization on the sieving properties of the membrane. In addition, it was found that recovery of the transport inhibitor was reduced by the polypropylene membrane support supplied by the manufacturer of the ultrafiltration cell. The evidence indicates that the inhibitor adsorbs to polypropylene. The net effect of these two variables on ultrafiltration can result in both false positive and false negative bioassays for the humoral sodium transport inhibitor.