The aim of this study was to determine 2-chloro, and 2-bromo derivatives of 2'-deoxyadenosine (2-CdA and 2-BdA respectively) in the serum of human blood by high performance liquid chromatography (HPLC). 2-CdA is a substance with anticancer and immunosuppressive activity. Caffeine or 5,6-dimethylbenzo-1,2,4-triazole-1-beta-D-ribofuranoside was added to 1 cm3 of serum as internal standard. 2-CdA and 2-BdA were isolated from serum using acetone as deproteinizing reagent. LiChrosorb Si 60 column was used for the separation of drugs and the internal standard from endogenous compounds in the sample. A mixture of potassium dihydrophosphate-methanol was used as a mobile phase.