Diazepam binding studies with dextran 70 and human serum albumin (HSA) were carried out using centrifugation and a membrane ultrafiltration technique to separate the drug protein complex and the free microsolute. The molecular filtration method was found to be precise and reproducible with very little variations. The binding of diazepam to dextran was significant but considerably smaller when compared with the extensive binding of diazepam to HSA. The results of these studies demonstrate that diazepam binding of HSA is altered in the presence of dextran. Diazepam binds to dextran resulting in displacement of diazepam from albumin binding sites. The contributing effect of dextran to overall diazepam binding increased as the dextran concentration increased and the HSA concentration decreased. However, the net result of HSA dilution with dextran 70 was still an increase in the percent free diazepam. Various concentrations of dextran 70 significantly displaced HSA bound diazepam in all the mixtures studied.