Friend leukemia cells from exponentially growing or differentiated (DIMSO-induced) cultures were permealized and their DNA was stained with 4′6-diamidino-2-phenylinciole (DAPI), Hoechst 33342, acridine organge, ethidium bromide, propidium iodide, Quinacrine, 7-amino-actinomycin D, mithramycin, or chromomycin A3. Accessibility, of DNA to each of the above fluorochromes was compared in differentiated and nondifferentiated cells before and after nuclear proteins, mostly histones, were extracted with O. 1N HC1. A decrease in the accessibility of DNA to several dyes, especially pronounced in the case of some intercalators, was observed in differentiated cells. After extraction of nuclear proteins with HO there was an increase in DNA accessibility, of varying degree depending on the fluorochrome and the difference between differentiated and nondifferentiated cells was abolished for most of the intercalating dyes. The increase was the lowest for DAPI (45%), the highest for 7-amino-actinomycin D (13 fold), and in general was higher for the intercalating dyes that unwind DNA than for dyes binding externally to the double helix. The results are discussed in terms of the mode of interactions between DNA and the fluorochromes and factors associated with chromatin structure that may affect accessibility of DNA in situ in exponentially growing and differentiated cells.