The paper presents research carried on biosorption of heavy metals mechanism recognition, particularly Cr3+, Cd2+ and Cu2+, carried by a blue-green alga Spirulina sp. cultivated in photoautotrophic and mixotrophic and manner, as well as cell lyophilisate. 4 hypothetical biosorption mechanisms were considered: sorption, microprecipitation, ion exchange and metal uptake into the cell's interior. Performed experiments revealed dominating mechanism of ion exchange.