The plasma concentrations of alpha 1-acid glycoprotein (AAG) and albumin, and the plasma protein binding of propranolol were measured in a group of cancer patients and a group of normal subjects. In cancer patients the AAG concentrations were twice that in controls (142 and 78 mg/dl, P less than 0.005), the albumin concentrations were lower (3.11 and 4.37 gm/dl, P less than 0.001), and the free fraction of propranolol was lower (0.127 and 0.190, P less than 0.005). Propranolol binding correlated strongly with AAG concentrations. These data imply that untreated or unsuccessfully treated cancer patients will have reduced free fractions for any drug for which AAG is an important binding protein. Successfully treated patients may have longitudinal changes towards normal. No consistent effect of the treatments themselves on AAG was observed after radiation therapy in five patients or after intravenous adriamycin in six patients.