In the last decade, suramin has become known for its antiproliferative, differentiation-inducing effects on cells and has been successfully used in the therapy of cancer patients. The present study was undertaken to investigate the effects of suramin on normal human thyroid cells in primary monolayer culture and to analyse whether it also affected cells from thyroid carcinomas. The results show that suramin, at concentrations similar to serum levels obtainable during therapy, inhibited the proliferation of thyroid cells as well as the secretion of thyroglobulin. It suppressed the activation of adenylyl cyclase in thyroid membranes and decreased the immunogenicity of the cells by reducing their surface expression of HLA-DR and ICAM-1. Although the morphology of differentiated thyroid cells remained unaffected by suramin, morphological changes compatible with differentiation were observed in cells from undifferentiated thyroid carcinomas when suramin was added to the culture medium. In conclusion, the data demonstrate that suramin has pronounced in-vitro effects on normal and neoplastic thyroid cells. It may, therefore, also be effective in patients with thyroid cancer, for whom no other form of treatment is available.