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The rate of reaction of monochlorobimane with glutathione (GSH) was measured in native human mammary MCF-7 adenocarcinoma cells (MCF-7wt) and sublines displaying resistance to 4-hydroperoxycyclophosphamide (MCF-7hc) and adriamycin (MCF-7adr) prior to examination by epifluorescence and confocal microscopy. After a 60-min incubation period at 37 degrees C, essentially all GSH was conjugated in the MCF-7wt and MCF-7adr cell lines whereas only 80% of the GSH was conjugated in the MCF-7hc line. All three lines displayed significant export of the conjugate from the cell during this period, with the MCF-7adr line displaying the most rapid efflux with 85% of the conjugate exported within 60 min. Epifluorescence microscopy detected an approximately 20% increase in integrated fluorescence intensity in the nuclear region in all three lines. Confocal microscopy however, indicated that most of the cells examined showed a homogeneous fluorescence distribution. The cells grown in monolayers were found to be thicker in the nuclear region suggesting that the observed increase in fluorescence intensity in the nuclear region in the images from epifluorescence microscopy was probably derived from fluorescence from an out-of-focus plane. Cells depleted of GSH with buthionine sulfoximine followed by treatment with mBCl showed significant fluorescence intensity resulting from nonspecific binding of this probe. These studies illustrate the need for measuring the rate of GSH conjugate export and for determining probe specificity, and emphasizes the need for using confocal techniques for the quantitative evaluation of the distribution of intracellular fluorescence. [ABSTRACT FROM AUTHOR]