The unicellular ciliateTetrahymena pyriformiswas treated with different concentrations of insulin or histamine and at different time points the cell density was measured, using a tetrazolium-based semiautomated colorimetric assay (MTT). The assay was suitable to determine the rate of cell proliferation of Tetrahymena. Insulin in each concentration significantly elevated the cell count up to 3h. After that, it was neutral or its effect was insignificant. Histamine at 10−5m concentration diminished cell count at 3, 5, 7 and 24h. At 10−6m concentration there was no difference and at 10−7m concentration it enhanced cell division up to 5h, after that there being no difference. The two hormones have cell division promoting activity for cells of higher animals and the experiments demonstrate this effect already at a unicellular level.