From a total of 418 primary amniotic fluid colonies, 5.5 % were fibroblast‐like (F), 33.7 % epithelioid (E) and 60.8 % had characteristics of what was previously shown to be the principal class of clonable amniotic fluid cells (AF). Polyploidy occurred in all three categories, although both pure tetraploidy and mixoploidy were more frequent in E colonies. The incidence of non‐constitutional chromosomal changes was identical in AF and E type colonies if primary spreads were analyzedin situwithout prior trypsinization. Spreads from pooled cellsuspensionsshowed higher base‐line levels of both aneuploidy and structural changes. Analysis of individual colonies employing anin situpreparative technique is clearly the method of choice for a reliable cytogenetic prenatal diagnosis within the shortest possible period of