SummaryRed blood cell S‐adenosylhomocysteine hydrolase (AHCY) from individuals of 1, 2‐1 and 3‐1 phenotypes was partially purified andKmandVmaxdetermined in the absence and in the presence of the following inhibitors: 3‐deaza‐adenosine (DZA), 3‐deaza‐aristeromycin (DZAry), 2–chloro adenosine (2‐Cl‐ado) and purine riboside (or nebularine). The three phenotypes 1, 2‐1, 3‐1 showed similarKm(32·58, 39·22 and 34·84 μmrespectively), but the ratioKm/Vmaxwas statistically different. DZA and DZAry appeared to be strong competitive inhibitors. The AHCY 1 phenotype was more resistant to their action, while the 3‐1 variant was more sensitive. 2‐Cl‐ado and purine riboside were weaker inhibitors; the type of inhibition varied among the three phenotypes, but, again, the AHCY 1 phenotype was