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We established a cell line (hPTC) from the tissue of papillary thyroid cancer surgically excised from a 27-year-old female patient. Synthesis of cAMP by the hPTC cells was stimulated by TSH. This cell line has continued to divide as a monolayer in a tissue culture for three years. We assessed growth regulation of the hPTC cells by protein tyrosine kinase and cAMP-dependent protein kinase by measuring the DNA content of the hPTC cells in 24-well plates with 3,5-diaminobenzoic acid after incubation in various growth factors. Basic fibroblast growth factor (FGF), epidermal growth factor (EGF), and insulin-like growth factor-1 (IGF-1), all of which bind to their respective receptors with tyrosine kinase activity, stimulated DNA synthesis in the hPTC cells. Neutralizing antibodies to basic FGF and EGF suppressed the growth stimulation by basic FGF and EGF, respectively. Genistein, a specific protein tyrosine kinase inhibitor, inhibited proliferation of the hPTC cells. On the other hand, thyrotropin, dibutyryl cAMP (dBC) and forskolin inhibited proliferation. KT5720, a specific cAMP-dependent protein kinase inhibitor, restored the growth of the hPTC cells even in the presence of dBC. This study shows that stimulation of the protein tyrosine kinase activity by basic FGF, EGF, and IGF-1 promoted DNA replication by the human thyroid cancer cell line. However, activation of the cAMP-dependent protein kinase inhibited proliferation of this cell line.