Abstract
Interphase nucleoli of Ehrlich ascites tumor cells were stained with amido black 10B to the exclusion of all other regions of interphase and mitotic cells. Proteins of the ascites fluid also remained unstained. Selectivity of the dye for nucleolar protein, established by blocking and extraction methods, is metal-mediated and involves amino groups predominantly. The conditions leading to selectivity are narrowly circumscribed in Ehrlich ascites tumor cells and, thus far, have not yielded equally satisfactory results with nontumor material to warrant general applicability of the method. A tentative explanation of the phenomenon is offered on the basis of numerous control experiments involving freezing-drying, variations in composition and application of reagents and other pertinent cytochemical procedures. The findings suggest that nucleolar proteins possess certain unique properties which are lost on altered to the extent that they are insensitive to the amido black 10B method once the proteins are released from the interphase nucleolus.