Previous studies of human congenital immunodeficiency states and in vitro observations of lymphocyte response to mitogens have implicated two purine salvage pathway enzymes, andenosine deaminase (ADA) and nucleoside phosphorylase (NP), as critical in the normal maturation and/or function of the immune system. Based on this information, ADA and NP activities were examined in a variety of congenital and acquired animal models of dysimmunity. The animals studied herein included: congenitally athymic (nude) mice; congenitally asplenic mice; congenitally athymic-asplenic mice; motheaten mice; New Zealand mice; and Arabian foals with severe combined immunodeficiency. No significant differences in the activities of ADA and NP were observed in any of these animals when compared with either normal littermates or animals with intact immune function. Major species differences were apparent when erythrocyte ADA acitivty was compared between mice and horses. In contrast, only minor strain alterations in ADA or NP activity were noted between several inbred groups of mice.