This study was conducted to investigate the influence of biochanin A (isoflavone characterised by estrogenic activity) upon the growth expressed as number of cells and the content of chlorophyll a and b, total carotenoids, reducing sugars, water-soluble proteins and nucleic acids (DNA, RNA) in the unicellular green alga Chlorella vulgaris, as an experimental model. Biochanin A in a concentration of 10-6 M exerted the greatest biological activity, principally, on the 6th day of cultivation, whereas during the 9th day of algal culure it had weak stimulating properties. Under the influence of 10-6 M biochanin A, an increase in the number of cells to the level of 186% and the content of water-soluble proteins to 255%, reducing sugars to 505%, in comparision with the control culture (taken to be 100%), was observed. Moreover, the content of DNA was intensively stimulated in the range of 184% and RNA content reached the value 202% in regard to the control. Among the photosynthetic pigments, stimulation of the content of chlorophyll a to the level of 191%, chlorophyll b to 180% and total carotenoids to 172%, compared with the control culture of algae devoid of biochanin A, was recorded.