A methodology is presented for systematic analysis of purine enzymes in small lymphocyte subfractions. For the determination of 7 different enzymes of purine metabolism (hypoxanthine-guanine phosphoribosyltransferase (HG-PRT), adenine phosphoribosyl-transferase (A-PRT), adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP), adenosine kinase (AK), 5′-nucleotidase (5′N), and AMP-deaminase) less than 200,000 peripheral blood lymphocytes are needed. 1000–6000 lyophilised lymphocytes are incubated in micro-incubation vessels (3μl) with radioactive substrates for 15–180 min. Separation of substrates and products is achieved by thin-layer chromatography on PEI-cellulose. Addition of BSA to the incubation mixtures results in higher specific enzyme activities and narrower ranges of mean values of a control group.