SINJ is a new continuous human cell line derived from a lymph node metastasis of a probably sporadic medullary thyroid carcinoma. It is compared to MTC-SK, another medullary thyroid carcinoma cell line, established earlier (1). SINJ has been continuously cultivated in vitro for two years. The cells grow as a suspension of single cells and cell clusters. Repeated immunocytochemistry showed positive immunoreactivity with antibodies to CT, CGRP and GRP. The maintenance of NSE and chromogranins were proved. Northern blot analysis confirmed endocrine activity at mRNA level. Flow cytometry of 27 SINJ - clones showed 25 diploid and two tetraploid subpopulations. Cytogenetic analyses strengthened these findings. According to DNA analysis the cells are free of SV40 sequences. Tumorigenicity was proved in nude mice. The new cell line SINJ has potential use for in vitro studies of medullary thyroid carcinomas in sporadic as well as hereditary forms - the MEN2A syndrome.