ALTHOUGH pigment production by Monascus spp. in chemically defined media is well documented (in submerged cultures and free cells), very few information is available about the use of agro-industrial wastes and immobilized cells. In this study immobilized irradiated spores (in sponge cubes) of M. purpureus (24 h age and 0.5g cubes/50 ml medium) produced high amount of red pigment reached up to 2.32g/ 1, after 4 days of incubation, compared with the amount'of pigment produced by the free cells (1.84 g/ 1). Also, potato processing wastewater (PPW) was examined as the main culture medium for red pigment production by this fungus • under optimizing culture conditions for repeated batches. The results showed that with irradiated immobilized cells, the maximum amount of red pigment production (1.96 g/ 1) was recorded at the second batch. Moreover, high reductions of biochemical oxygen demand (BOD); 82.6 % for this waste was obtained during the second batch. The data revealed that very little amount of soluble toxic substances in the extracted sample leading to only 8% dead chicken embryos. Keywords: Monascus sp., pigments, submerged fermentation, gamma rays.