HGPRT from patient M. Y. (enzyme level 0.1% normal) retained a normal apparent K m both for PRPP and for hypoxanthine and was inhibited by its product. The enzyme was, however, unstable at 50 C (43% of activity remaining after 1 hr) when compared with normal controls (81% of activity retained). The enzyme from patient J.D. (enzyme level 0.005% normal) was also unstable (32% of activity retained). Unlike for M.Y., however, all the other characteristics studied were also altered. The enzyme activity was enhanced rather than inhibited by its product (IMP), and the apparent K m (hypoxanthine) could not be calculated due to the sigmoid nature of the curve. Obviously, there is marked heterogeneity in the nature of the biochemical lesion responsible for the Lesch-Nyhan syndrome in these patients, and this is discussed.