A factor produced by plerocercoids of the tapeworm Spirometra mansonoides is similar to human growth hormone (hGH) in that it stimulates body growth, binds to hGH receptors, cross-reacts with anti-hGH antibodies, and has lactogenic and insulinlike activities. The purpose of this study was to determine whether plerocercoid growth factor (PGF) is similar to hGH in expressing diabetogenic activity in the genetically obese (oblob) mouse. To determine an effective dose for use in the obese mice, the ability of daily injections of PGF to stimulate growth of phenotypically normal mice of the same strain was assessed in a 10-day weight gain assay. Injections of PGF stimulated a dose-dependent weight gain ( r = 0.83) and 25 ng eq/day of PGF stimulated a response not significantly different from that produced by 100 pg of bovine growth hormone/day. Diabetogenicity was assessed using fasting blood glucose and glucose tolerance tests in obese mice that had been injected for 3 days with saline, hGH, or PGF. Human growth hormone caused a significant increase (P e 0.005) in fasting blood glucose and glucose tolerance of the obese mice was impaired (P e 0.01). All of the doses of PGF used to test diabetogenicity in the obese mice were at least twice that required to stimulate a maximal growth response in normal mice, yet none of the doses of PGF increased fasting blood glucose or decreased glucose tolerance. These results show that PGF was a potent growth stimulant but was not diabetogenic. [P.S.E.B.M. 1989[P.S.E.B.M. , Vol 1911 t is well known that chronic administration of high doses of growth hormone (GH) to susceptible ani-I To whom requests for reprints should be addressed