Cremophor EL (CrEL) is a polyoxyethylated castor oil surfactant used in the intravenous formulation of the anticancer drug paclitaxel (Taxol). Quantitative determination of CrEL in patient samples can be achieved by complexation of the compound with the Coomassie brilliant blue G-250 dye in protein-free extracts [Sparreboom, A., Loos, W. J., Verweij, J., De Vos, A. I., Van der Burg, M. E. L., Stoter, G., and Nooter, K., Anal. Biochem. 255, 171-175 (1998)]. A disadvantage of this method of CrEL determination is that the assay plot of absorbance at 595 nm, the peak wavelength of the CrEL-dye complex, versus the concentration of the surfactant is not linear. The present study shows that the nonlinearity is associated with a decrease in the free dye concentration and a reduction in complex formation by increasing the CrEL concentration. By measurement of the ratio of absorbances at the maxima of the red (450 nm) and blue charge forms (595 nm) of Coomassie brilliant blue G-250, a full-scale linear relationship can be obtained over the entire range studied (0.500 to 10.0 microliter/mL). Validation data revealed that transformation of the detection procedure exhibits significantly improved specificity, accuracy(</= 6.33% relative error), and precision (< 10.0%) compared to our previous assay. The modified method was successfully applied to the measurement of CrEL in plasma of 11 cancer patients treated with a 1-h infusion of paclitaxel.