Different substances of amniotic fluid influence the cell proliferation and differentiation of developing animal fetus. The aim of this study was to determine the mitogenic effect of some peptide components of bovine amniotic fluid on bovine peripheral blood lymphocytes using methyl tetrazolium (MTT) colorimetric assay. The next aim of our work was to determine the mitogenic activity of ovine amniotic fluid fractions of peptide nature on benzo-á-pyrene transformed BALB/c 3T3 mouse fibroblasts (BPA31 cells) by use of 3H-thymidine incorporation into nucleus DNA and in conclusion, to compare the mitogenic activity of ruminant amniotic fluid fractions on different indicator cells. According to our study, inhibiting effect was found only in the case of separated bovine amniotic fluid (Peak I) and ovine amniotic fluid (B fraction). On the other hand, we have observed activation of lymphocytes by other fraction of bovine amniotic fluid (Peak II) and also of BP- A31 cells by fraction A in case of ovine amniotic fluid. The proliferation of peripheral lymphocytes was not significantly changed after the addition of natural bovine amniotic fluid likewise when the delipidated ovine amniotic fluid was added to BP-A31 cells, there was no effect on 3H-thymidine incorporation. Our results suggest that with testing the proliferation effect, the selection of indicator cells is of great importance since various cell types respond in different ways to the same substances.