SummaryA colorimetric method was used to study the variations in the haemolytic rates between red cells from different individuals and with different blood factors. Marked differences were observed between the heterozygous and homozygous genotypes of 18 out of 20 different blood factors and between the B1or E3factors, when they occurred in heterozygous genotypes but in different phenogroups. Between MZ twin pairs a continuous variation was found in the haemolytic rates for 17 out of 18 different blood factors, which indicated a quantitative genetic variation depending on individuality, in addition to the dosage‐ and pheotype‐dependent variation.The similar rankings of the haemolytic rates of the blood factors of the B1O3Y2A'E3phenotype in red cells from 11 MZ twin pairs suggested their simultaneous regulation. When the haemolytic rates of different blood factors at four or more loci were compared for 15 MZ twin pairs, the ranking results of a given locus were independent of those of the other loci in all pairs, except