Routine cytogenetic analysis of an amniotic fluid sample revealed a large brightly fluorescent region in the short arm of chromosome 14 in an otherwise normal male karyotype (46, XY, 14p+++). This site was also present in the paternal karyotype. In situ hybridisation to a Y‐chromosome‐specific DNA probe confirmed that the father had a Y/14 translocation. The incidence of two hybridisation bodies (large hybridisation sites), detecting both the translocated Y chromatin and the normal Y chromosome, was lower in interphase nuclei (44.3%) than in metaphase spreads (95.2%). The relevance of these observations to the potential use of in situ hybridisation to intephase nuclei for prenatal diagnosis is discussed.