Dissolved organic matter in the form of riverine humic substances stimulated the growth of both axenic nitrogen-limited Alexandrium catenella cultures and nitrogen-limited cultures with a marine bacterial community present. The biomass increase of A. catenella could not be accounted for by utilization of inorganic nitrogen compounds. However, there was a considerable release of dissolved free and combined amino acids from the humic substances that was utilized by A. catenella. About 40% of the nitrogen used by A. catenella in the axenic treatment with humic substances added was taken up as organic nitrogen. Bacterial aminopeptidase and beta -glucosidase activity was stimulated by the addition of humic substances and bacterial growth increased several-fold. Bacteria also utilized the released amino acids from the humic substances, but did not remineralize nitrogen, since no increase in ammonium concentrations could be detected in the bacteria treatments with humic substances added. In the axenic A. catenella treatment there was no significant aminopeptidase activity, suggesting that A. catenella was able to utilize the dissolved combined amino acids directly. Moreover, large fluorescently labeled dextran molecules (2000 kDa) were taken up by A. catenella in the humic treatments, showing up in vacuoles inside the cells. These results suggests that A. catenella can grow well utilizing macromolecular organic compounds containing nitrogen, probably by a direct uptake.