Typing of both SAHH and ADA red cell electrophoretic patterns was carried out among the members of about 80 families from Latium (Central Italy) and in a random sample of about 350 individuals from two Italian regions, Latium and Sardinia. 1. The SAHH1 enzyme product provided another interesting example of a change in the electrophoretic pattern brought about by the haemolysate ageing. In vitro storage of SAHH 1 red cell lysates leads to the production of a pattern similar to that expected from a heterozygote SAHH 2-1. This change has been shown to be abolished by pretreating the sample with mercaptoethanol. The results indicate that the systematic use of sulphydril reducing agents can provide a more reliable means of analysing the SAHH polymorphism if differently stored samples are to be compared by starch gel electrophoresis. 2. Evidence against complete linkage of the SAHH and ADA loci has been obtained from two informative SAHH/ADA matings encountered in this study. 3. The SAHH allele frequencies observed in the two samples analysed were: SAHH1 = 0.969, SAHH2 = 0.024, SAHH3 = 0.007 (Latium) and SAHH1 = 0.973, SAHH2 = 0.011, SAHH3 = 0.016 (Sardinia). 4. The ADA2 allele frequency estimates were: 0.083 (Latium) and 0.059 (Sardinia). These figures are almost identical to those already reported for the same two regions.