Authors described genetically determined hereditary polymorphisms and pattern of inheritance of the adnosindesaminase (ADA) system. After a methodical review, results of their own investigations are reported, which were the first in the Hungarian population. The incidence of the ADA phenotypes among 1234 adult persons investigated was found: ADA-1=88%, ADA 2-1=11,58%, ADA 2=0,42%. Rare phenotypes were not detected. Values of gene frequency found: ADA1=0,93,79; ADA2= 0,0621; Results of the investigation of 561 mother-child pairs are also reported. The pattern of inheritance in every case examined was found to be regular. According to the author's data the maximal (theoretical) chance of the exclusion of paternity was 5,48%. In 561 cases of discussed paternity beside the examination of the systems of A1A2BO, MN, Ss, Hp, Rh, Gm(a,x,b), INV (1), Gc, Kk, Fy(a,b), SEP, PGM, GPT, and AK the ADA system was also examined. On the base of the last examinations the paternity could be excluded in 11 (1,97%) cases. In two cases (0,36%) exclusion of the paternity was based on the results of the examination of the ADA-system only. In the biostatistical evaluation of the results of examinations of blood-groups the role of the ADA system seems to be important. Examination of ADA system in cases of disputed paternity is recommended.