Haematococcus pluvialis was cultured under N- and Mg+2-deficient conditions with two light intensities : 40 and 230 mu mol m(2) s(-1). Highest astaxanthin concentration. 49.5 mu g.ml(-1), was obtained when high light was applied under N-deficient conditions. N-deficiency has a greater effect than high light intensity on astaxanthin synthesis by exerting a stronger blocking effect on cell division. The effect of high light was synergetic with the other stress conditions in stimulating the synthesis of astaxanthin. Mg+2 deficiency also stimulated the synthesis of astaxanthin but produced lower concentrations : 7 and 26 mu g.ml(-1) for low and high light intensities respectively. When both N and Mg+2 were absent from the culture media the concentration of astaxanthin was lower than with N-deficiency alone but higher than with Mg+2-deficiency.