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The in vitro interaction between purified bovine liver and sperm DNA with somatic histones, to form nucleosomes, and with bovine and salmon protamines were studied. DNAse or microccocal nuclease digestion of liver DNA-histone reassociated chromatin produced the expected polynucleosome type of fragments. Electrophoretic patterns of digested sperm-DNA nucleosomes were different. Micrococcal nuclease digestion produced mainly fragments smaller than 100 bp and some nucleosome-type particles. Under DNAse activity most of the products were smaller than 100 bp, indicating an increased susceptibility of the sperm DNA-histone complexes to the hydrolytic activity of both nucleases, particularly toward DNAse I. This differential susceptibility was confirmed by sucrose gradient spectrophotometric analysis. Acridine orange (AO) staining of histone-DNA reassociated nucleosomes showed significant differences in fluorescence intensity, sperm DNA-histone complexes being almost twice as fluorescent as liver DNA-histone complexes. On the contrary, liver DNA/protamine complexes stained with AO were consistently more fluorescent than sperm DNA-protamine complexes. Finally, no differences in either fluorescence intensity or spectra were observed when liver and sperm DNA were stained with AO after interaction with salmon protamines. The data suggest that sperm DNA has important structural characteristics that differentiates it from somatic DNA. These differences seem to be species specific and must surely play an important role on the determination of the dramatic sequence of that participates sperm chromatin organization. [ABSTRACT FROM AUTHOR]