The binding of cephalothin sodium, phenobarbital sodium, dexamethasone sodium phosphate, isoproterenol hydrochloride, digoxin, dactinomycin and phenytoin sodium to three intravenous inline filters--a 5-micrometer stainless steel depth filter, a 0.2-micrometer cellulose ester membrane and a 0.2-micrometer polycarbonate membrane--was studied. The experiments were conducted simulating inline i.v. filtration of these drugs in three i.v. solutions--lactated Ringer's, 5% dextrose and normal saline. Cephalothin was assayed colorimetrically, phenobarbital and phenytoin spectrophotometrically, and the other drugs by radiotracer technique. Binding of the drugs to the filter was found to be insignificant from a therapeutic standpoint, except for dactinomycin which bound to the 0.2-uicrometer filters. The binding of dactinomycin was approximately 13% of the amount administered through cellulose ester and polycarbonate membranes, respectively. The binding occurred during the initial period of filtration of the drug solution. It was concluded that inline filtration of drugs administered in relatively high does should not present any problem concerning the reduction of the therapeutic potency because of filtration.