Zidovudine (AZT) is widely used in the treatment of human immunodeficiency virus (HIV) infection because of its ability to act as a false substrate for the viral reverse transcriptase. AZT in the clinical setting has also been shown to produce significant hematologic and other toxicity, even though prior reports in the literature indicate that AZT is not readily incorporated into mammalian nuclear DNA. The human myeloid leukemia cell line HL-60 was grown in the presence of radiolabeled AZT or radiolabeled deoxythymidine as an in vitro model to study AZT toxicity for human myeloid cells. Radioactivity was reversibly incorporated into the mitochondria, suggesting that AZT toxicity may result in part from its toxic effects on human mitochondria rather than on human nuclear DNA.