One percent of the Algerian population carries a weak allele at the DIA1 locus, responsible for a 50% decrease of red cell soluble cytochrome b5 reductase activity. Quantitative abnormalities of the soluble and the membrane-bound enzyme have been investigated in the red cells and in the leukocytes of seven subjects considered to be heterozygous at the DIA1 locus. Conventional electrophoretic or isoelectrophoretic studies did not show any qualitative abnormality. However, continuous titration obtained by combined IEF-electrophoresis displayed in five out of seven subjects a discrete abnormal line on the titration curve compatible with an Arg leads to His substitution. In fact, at least three types of weak alleles could be defined by combining the qualitative and quantitative results obtained with the erythrocyte (soluble and membrane-bound) and leukocyte enzyme. We call these subgroups DIA Mustapha1, DIA Mustapha2, and DIA Mustapha3.