Natural humate from compost and the model sunstance obtained from p-benzoquinone were dissolved in an acetone-water mixture and subjected to chromatography on a column of aluminium oxide. Similar fractions were obtained which were chemically and spectrophotometrically investigated for the content of functional groups. The particle size of these substances was determined by filtration on Sephadex molecular sieves. Experiments were performed with tomato in water cultures on stagnant nutrient solution. The biological activity of the corresponding fractions of natural and model humates was found to be analogous. The results are discussed and confronted with functional groups content and particle size of the respective substances.