Six clonal lines of human choriocarcinoma cells, isolated by the single cell plating technique, have been maintained in culture for over two years. These clones have been examined for expression of tissue-specific function. Individual clonal lines have retained the capacity to synthesize human chorionic gonadotrophin (HCG), human chorionic somatomammotrophin (HCS), and progesterone, and also to transform steroid precursors to oestrone and oestradiol. The HCG species produced by different clones have different biologic to immunologic potency ratios ranging from 0.5 to 2.0 which are constant within any particular clone. HCG synthesized in culture has been examined by polyacrylamide gel electrophoresis and isoelectric focusing and had approximately the same molecular size as a highly purified urinary HCG preparation, but a different net charge at an acid pH.