In peripheral human blood lymphocytes the uptake and metabolism of adenine, guanine, and hypoxanthine was investigated. This was achieved by incubation of purified lymphocytes with 14C-purine bases, separation of cells from the incubation medium by a rapid filtration technique, and subsequent separation of the acid soluble material by thin-layer chromatography. No perferential uptake for one of the purine bases was observed. In all cases only traces of 14C-purine bases not added originally and labeled nucleosides could be demonstrated. Approximately 2/3 of adenine and 1/2 of guanine or hypoxanthine were converted to nucleotides. Separation of formed nucleotides showed that adenine and guanine were metabolized mainly to their corresponding nucleotides; hypoxanthine was converted to a considerable amount to adenine nucleotides and only to a small proportion into its own nucleotides. These results demonstrate the predomonance of adenine nucleotide formation in normal human lymphocytes.