The study demonstrated that nuclei and mitochondria became invisible in the keratinizing hair shaft at about the same time. This was found to occur fairly early in the process at the level of the hair shaft where the hair cuticle becomes permanent. The lower nuclear DNA (nuDNA) copy number and higher degree of keratinization in hair shafts are probably the major factors in hair-shaft nuDNA extraction failures. Mitochondrial DNA (mtDNA) in hair shafts is easily recovered several centimeters beyond the location in the keratinized hair shaft where mitochondria are no longer visible. Apparently the mitochondrion provides no additional protection for mtDNA in the keratinized hair shaft. For this study, brown Caucasian anagen head hair shafts without follicles were obtained from one person by plucking. The hairs were immediately placed in a fixative solution of 3-percent paraformaledyde and 1.5 percent glutaraldehyde in phosphate buffer for at least 1 day at 4 degrees C. After postfixation in 1-percent osmium tetroxide, the hairs were dehydrated in a series of alcohols, and propylene oxide was used as a transition solvent prior to embedment in 100-percent epoxy resin. Further details are provided on procedures for conducting transmission electron microscopy and scanning electron microscopy. 5 figures and 11 references