BIOLOGICAL CONTROL MECHANISMS OF MARINE DINOFLAGELLATES PROLIFERATION I-GROWTH FACTORS AND MITOGENES. - The effects of growth factors and mitogenes (fetal bovine serum, platelet derivated growth factor and phorbol 12 monoacetate) on dinoflagellate proliferation were analyzed in axenic clonal cultures of Prorocentrum triestinum and Prorocentrum lima. According with the hypothesis concerning a universal mechanism controlling the cell division cycle in all the eukaryotic cells, apparently dinoflagellates respond to growth factors like mammalian cells do: growth factors significatively increased cell yield and acclimated maximal growth rates of dinoflagellates due to their specific mitogene effect. Apparently under growth factor supplementation dinoflagellate's growth is at a maximal, and their proliferation in culture medium derivated of natural sea water with f/2 nutrients enriched was simmilar to the growth in artificial ASPM f/2 culture medium (where traditionally dinoflagellates are able scarcely to grow). Results suggest that growth factors could play a significant role in the occurrence and proliferation of dinoflagellates blooms.