Human mitochondrial transcripts have been examined at the ultrastructural level. After contact with ultrathin sections of a human lymphoid cell line (CEM) embedded in Lowicryl K4M, biotinylated mitochondrial probes yield specific hybrids identified by a colloidal gold immunocytochemistry marker that visualizes rRNA and mRNA coding for respiratory chain polypeptides CO II, CO III and ATPase-6. The mitochondrial transcripts are preferentially located close to the inner membrane, particularly the cristae, suggesting that intra-organelle protein synthesis is intimately associated with the mitochondrial membrane system. Quantitative analysis indicates that the mitochondria concentrate the labeling with intensities that vary with the type of RNA and that the nucleus induces a light hybridization signal with each mitochondrial probe. The visualization of human mitochondrial DNA expression in correlation with the fine anatomy of the mitochondria constitutes a new approach for fundamental research on the organelle and for analyzing its behaviour in human mitochondrial diseases.