Relationship between callus size and plant regeneration in rice anther culture is described. There was no relation between the callus size and green plant regeneration. Regeneration of albino plants increased in proportion to the callus size in Nipponbare. The number of haploid green plants regenerated from calluses increased when the callus was > 2.0 mm in diameter. Spontaneous doubling of the chromosome number of green plants was promoted when the callus was 1.0-1.2 mm in diameter. The ratio of green plant regeneration (green plant/albino plant) and spontaneous doubling frequency of the chromosome number of green plants were higher in Akenohoshi than in Nipponbare, which may be related to the fact that the callus sizes of Akenohoshi were relatively small compared with those of Nipponbare. The effect of low temperature treatment of anthers on callus formation, regeneration of green and albino plants from calluses, and spontaneous doubling of the chromosome number of regenerated plants was also described. Low temperature treatment of anthers resulted in the increase of the callus formation frequency in Nipponbare, but not in Akenohoshi, as well as the increase of green plant regeneration and decrease of spontaneous doubling of the chromosome number of green plants, while low temperature treatment did not affect albino plant regeneration.