Two unrelated males, a 43-year-old man with azoospermia and a 4-year-old boy with stature at the 10th centile, had similar karyotypes: 46,X,min. The minutes, present in all cells analyzed, stained weakly with G-, C-, and Q-banding methods. To elucidate their origin we used molecular techniques: In HaeIII digests of total genomic DNA from both individuals, no Y-specific reiterated sequences were detected. However, restriction fragment analysis with probe pDP31 demonstrated that the patients' DNA contained the Y-specific fragment. In situ hybridization with the same probe showed that these sequences were present on the minute chromosomes and have not been translocated elsewhere.