In 1961, a prestigious group of medical researchers called on their colleagues to stop using the language of 'Mongolism' to describe people with what we now call 'Down's syndrome' (or Trisomy 21). This call responded to new knowledge about the biological basis of Down's syndrome: rather than the product of racial degeneration, as had been hypothesized in the 19th century, the condition was the result of an extra chromosome, dubbed '21'. Yet, despite this plea, the terms 'Mongol' and 'Mongolism' continued in scientific use through the 1960s. Drawing on published and archival materials, I argue that the new knowledge about chromosomes did not rupture older patterns of scientific practice or interpretation, and with them, older terminological habits. The persistence of the language of Mongolism reflects the continuity of a network of older approaches to interpreting the condition within the community of human and medical geneticists, including an enduring diagnostic and interpretive technology, dermatoglyphics. Old networks were not supplanted; they were re-aligned.