Neocarzinostatin is a potent enediyne antitumor antibiotic complex in which a chromophore is noncovalently bound to a carrier protein. The protein regulates availability of the drug by proper release of the biologically active chromophore. To understand the physiological mechanism of the drug delivery system, we have examined the trifluoroethanol (TFE)-induced conformational changes of the protein with special emphasis on their relation to the release of the chromophore from holo-neocarzinostatin. The effect of the helix-inducing agent, TFE, on the all -sheet neocarzinostatin protein was studied by circular dichroism, fluorescence, and ¹H NMR studies. Using binding of anilinonaphthalene sulphonic acid as a probe, we observed that the protein exists in a stable, partially structured intermediate state around 45-50% TFE, which is consistent with the results from tryptophan fluorescence and circular dichroism studies. The native state is stable until 20% TFE, and is half converted into the intermediate state at 30% TFE, which starts to collapse beyond 50%. HPLC analysis of the release of the chromophore caused by TFE treatment at 0^oC suggests that the release process, which occurs below 20% TFE, does not result from an observable conformational change in the protein. Kinetic measurements of the release of chromophore at 25^oC reveal that TFE does stimulate the rate of release, which increases sharply at 15% and reaches a maximum at 20% TFE, although no major secondary or tertiary structural change of the carrier protein is observed under these same conditions. Our data suggest that chromophore release results from a fluctuation of the protein structure which is stimulated by TFE. Complete release of the chromophore occurs at TFE concentrations where no overall observable unfolding of the apoprotein is seen. Thus, the results suggest that denaturation of the protein by TFE is not a necessary step for release of the tightly bound chromophore.