Apoptosis is a distinct form of cell death characterized by internucleosomal cleavage of DNA, cell membrane blebbing, condensation of nuclear chromatin in the nuclear periphery, and the formation of apoptotic, condensed nuclear bodies. The finding of internucleosomal cleavage of chromatin, perhaps caused by endonuclease activation, has become accepted as a hallmark of this form of cell death. We describe the incidental and artifactual finding of internucleosomal cleavage of chromatin from kidney tissue from normal animals. Nephrectomy was performed in living animals, and renal tissue was digested with proteinase K in 10 mmol/L Tris, 1 mmol/L ethylenediaminetetraacetic acid (EDTA), 10 mmol/L NaCl, and 0.5% sodium dodecyl sulfate. Agarose gel electrophoresis of extracted DNA showed internucleosomal cleavage. Internucleosomal cleavage of DNA was not tissue specific but was evident also in liver DNA from a number of animals. Histologic examination of kidney tissue where DNA exhibited internucleosomal cleavage showed normal morphology, with no evidence of either apoptotic or necrotic cell death. Cleavage was not completely prevented by immediate freezing of kidney tissue in liquid nitrogen before DNA extraction, nor was it prevented by the addition of spermidine, of ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid, of phenylmethylsulfonylfluride, or by an increased concentration of NaCl to 100 mmol/L in the digestion buffer. Internucleosomal cleavage of DNA was mostly, although not invariably, inhibited by the use of a digestion buffer containing 10 mmol/L Tris, 25 mmol/L EDTA, and 100 mmol/L NaCl. "Apoptotic" chromatin changes (internucleosomal fragmentation) are not always associated with histologic evidence of apoptosis and may occur artifactually.(ABSTRACT TRUNCATED AT 250 WORDS)