Synthesis of hCG and its alpha subunit (hCG-alpha) by trophoblastic tumor cell lines (JEG-3, Reid, and BeWo) and by nontrophoblastic tumor cell lines [ChaGo and three HeLa lines (HeLa CCL2, HeLa S3, and HeLa 65)] was studied. cAMP, dibutyryl cAMP, and 8-bromo-cAMP induced, but 5-bromo-2'-deoxyuridine inhibited, the synthesis of hCG-alpha and hCG in trophoblastic tumor cells, Addition of 5-bromo-2'-deoxyuridine partially blocked the induction by dibutyryl cAMP in these cells.