Non-modified and chemically modified algal biomass of Chlorella vulgaris and Scenedesmus armatus were chosen to examine their Zn 2+ , Pb 2+ and Cd 2+ sorption activity. The dry biomass was chemically modified with 1M HNO3 in order to determine the influence of acid--treatment on the heavy metal ions removal rate. The heavy metal uptake process was found to be rapid and very efficient. The sorption process of heavy metals is highly pH-dependent. It influences heavy metal forms in the solution, and functional groups located on the cell surface. The effectiveness of biosorption depends also on the biomass concentration and its modifications. The highest adsorption capacities of both, Chlorella vulgaris and Scenedesmus armatus towards zinc, lead and cadmium ions were found for the lowest sorbent concentrations. Batch experiments showed that the differences between uptake by non-modified and acid-modified biomass are relatively small. It can be due to the fact that non-modified algal biomass exhibits high sorption capacity.