Mitochondrial DNA (mt DNA) molecules, isolated from normal human lymphocytes, lymphocytes stimulated with phytohemaglutinin (72 h) and a cultured Burkitt’s lymphoma cell line, were examined by electron microscopy. Only monomeric forms of mt DNA were found in normal human lymphocytes, whereas in the other cell types, catenated forms were also observed. Unicircular complex forms were apparently absent in all the cell types studied, suggesting that these forms are not a common malignant transformation in human cells. No clear correlation between mitochondrial modifications – as detected by stereological analysis – and the presence of catenated forms could be established. It is suggested that other mechanisms rather than replication of mt DNA would appear to be the mechanism responsible for the formation of these