Four human melanoma cell lines were examined for their responsiveness to the hormones 1,25-dihydroxyvitamin D3 (1,25[OH]2D3), calcitonin, and parathyroid hormone (1-34). Cells from each of the 4 lines contained high affinity binding sites for 1,25(OH)2D3. At high cell densities, binding of 1,25(OH)2D3 was diminished due to a decrease in receptor number with no apparent change in affinity. Preincubation with 1,25(OH)2D3 (10-10 to 10-8 M) increased tyrosinase activity 1.3- to 3.2-fold and 25-hydroxyvitamin D3-24-hydroxylase activity 1.4- to 10-fold. Human calcitonin (0.82 to 82.5 ng/well) raised the intracellular concentration of cyclic adenosine monophosphate 1.4- to 9.4-fold. Tyrosinase activity increased in response to calcitonin in 2 of the cell lines, decreased in the third, and showed no change in the fourth. Human parathyroid hormone (1-34) in concentrations of 1 to 10 ng/ml produced no significant changes in cyclic adenosine monophosphate accumulation, cell numbers, or tyrosinase activity in any of the cell lines. This study indicates that the phenotype of human melanoma cells can be modulated by the calciotropic hormones 1,25(OH)2D3 and calcitonin. © 1988.