A means of measuring in vivo mutations in man would be valuable in assessing the importance of mutation in ageing and human disease and for monitoring individuals exposed to environmental mutagens and carcinogens. The hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus has been used to study mutagenesis in cultured mammalian (and other) cells1; clones are selected by their ability to grow in the presence of the purine analogue, 6-thioguanine (6-TG). Such clones are almost always HGPRT- and are therefore probably the progeny of a mutant cell. This system has been applied to human lymphocytes by using autoradiography to detect single mutant cells 2-6. This approach probably does detect mutant cells, particularly if their number is increased, but the results obtained are variable and rather imprecise4,5. Also, the technique does not allow isolation of clones, thus the mutational origin of thioguanine-resistant (TGr) cells cannot be determined as the gene product, HGPRT, cannot be measured. Previous methods for cloning peripheral blood lymphocytes (PBL) have been so inefficient that detection of TGr lymphocytes has not been practicable. We have cloned PBL by a technique which is highly efficient (20-60% of PBL form a clone) and results in large clones of 103-104 cells that are readily scored using an inverted microscope6,7. We report here that the in vivo mutation frequency is of the order of that observed for human fibroblast lines cultured in vitro1.