Enzyme assays of skin fibroblasts from five children with familial hyperlysinemia from unrelated families are added to the previous report of three children from two unrelated families. In all instances there was a deficiency in lysine-ketoglutarate reductase, saccharopine dehydrogenase, and saccharopine oxidoreductase activities. To complete the studies on the enzymes associated with familial hyperlysinemia, saccharopine oxidoreductase was partially purified from human liver and characterized. The activity did not separate from that of lysine-ketoglutarate reductase or saccharopine dehydrogenase. A simple screening test for familial hyperlysinemia is described based on the evolution of 14CO2 from lysine-14C by skin fibroblasts. The test differentiated, without overlap, seven patients with familial hyperlysinemia from control subjects. The relation of the two genetic entities involving lysine degradation, familial hyperlysinemia and saccharopinuria, is discussed. It is suggested that familial hyperlysinemia, type I, be applied to patients with major defects in lysine-ketoglutarate reductase and saccharopine dehydrogenase, and that familial hyperlysinemia, type II, to be used to designate patients in whom significant amounts of lysine-ketoglutarate reductase are retained. The nomenclature would be consistent with that of an analogous disease, orotic aciduria.