A microscale isoelectric focusing technique has been developed and used to study hypoxanthine-guanine phosphoribosyl transferase (HGPRT; E.C. 184.108.40.206, inosinate-guanylate:pyrophosphate phosphoribosyl transferase) activities in mouse and human cell lines. The enzymes of both mouse and human origin are shown to exhibit considerable heterogeneity, but each type has a unique range of isoelectric pH. The enzyme of a mouse × human hybrid cell line, derived by fusion of HGPRT− parental cells, gives a homogeneous peak of activity, unlike the wild-type enzyme of either parent. The possibility is suggested that this enzyme activity is due to intra-allelic complementation.