Monolayer cultures of medullary thyroid carcinoma (MTC) cells were prepared from 5 patients with familial and 4 with sporadic MTC. Basal calcitonin (CT) release decreased rapidly during 17 days in culture. Addition of extra calcium (2 mmol/l) or dibutyryl cyclic AMP (dBcAMP, 3 mmol/l) increased CT release in all investigated cultures (p less than 0.01). No significant differences were observed between cell suspensions prepared from familial and those from sporadic MTC. Carcinoembryonic antigen (CEA) was found in 3 out of 5 MTC cell suspensions from patients with familial tumors and in 2 out of 2 sporadic tumors. Only two cultures secreted measurable amounts of CEA, which decreased with culture time but at a much slower rate than that of CT release. CEA release was not affected by the addition of calcium and dBcAMP. This study shows an absent relation between basal and stimulated CT and CEA release by cultured MTC cells.