When an initial cell loading of about 30-40 mu g chlorophyll (Chi).g(-1) gel and alginate suspension of 3% (w/v) were used for immobilization of Chlamydomonas reinhardtii, the resulting cell beads showed optimum nitrite uptake rate, at 30 degrees C and pH 7.5, of 14 mu mol NO2-.mg(-1) Chl.h(-1), the photosynthetic and respiratory activities being about 120 mu mol O-2 produced.mg(-1) Chl.h(-1), and 40 mu mol O-2 consumed.mg(-1) Chl.h(-1), respectively. The nitrite uptake activity required CO2 in the culture and persisted after 8 days of cells immobilization, or in the presence of 0.2 mM ammonium in the medium. Our data indicate that alginate-entrapped C. reinhardtii cells may provide a stable and functional system for removing nitrogenous contaminants from waste-waters.