Ammonium salts, especially ammonium nitrate, have been used as nitrogen sources for production of traditional water-insoluble Monascus pigments. However, we noted that defined media employing NH4NO3 as the sole nitrogen source in fermentations supported only poor pigment production by Monascus sp., and the pigments produced were mainly cell-bound. NH4NO3 was found not to (a) repress pigment synthase formation, (b) enhance synthase decay, or (c) serve as a nitrogen source for pigment production by resting cells; it had a weak inhibitory effect on the action of pigment synthase(s). The high level of cell-bound pigments accumulated in NH4NO3-grown cells did not exert a feedback effect on the further synthesis of pigments. These observations indicate that the reason why NH4NO3 supports only low pigment production during fermentations is the poor ability of NH4NO3 to donate nitrogen in the Schiff-base reaction converting-orange pigments to red ones.