The accumulation of administrated fluorescein in some breast diseases was investigated. The fluorescein concentration in about 200 macropreparations of resected mammas was measured with ratio fluorometer. The accumulation of dye in breast cancers is significantly different from that in benign diseases when determined in more than 10 hours after the dye administration. The critical value 1.75 of fluorescein accumulation factor sets the false positive error as well as false negative error to be equal to 6.6% for the fluorescent method of diagnostics. These data may be used both in express intra-surgical fluorescent diagnostics and in NMR imaging with chemically modified fluorescein.