The binding of taxol to human plasma and to individual plasma proteins was studied by equilibrium dialysis. Taxol was found to bind extensively (about 95%) without a significant difference between healthy volunteers and cancer patients. At clinically relevant concentrations (0.1-6 microM), the binding was found to be concentration independent, indicating nonspecific hydrophobic binding. Human serum albumin and alpha 1-acid glycoprotein were found to contribute about equally to the binding, with a minor contribution from lipoproteins. None of the drugs commonly coadministered with taxol (dexamethasone, diphenhydramine, ranitidine, doxorubicin, 5-fluorouracil and cisplatin) altered the binding of taxol significantly. The protein binding of taxol was found to dramatically decrease the red blood cell uptake of taxol.