A gonadotrophin with testicular interstitial-cell-stimulating activity was detected in extracts of hepatoblastoma and a pulmonary metastasis and in the urine of a 3-yr-old male with isosexual precocity manifested by advanced skeletal maturation (bone age 4 yr), adult urinary testosterone excretion (62 μg/day) and Ley dig cell hyperplasia. The gonadotrophin content of the hepatoblastoma, measured by the ventral prostate weight method in immature, hypophysectomized male rats, was 2.6 IU/g. This gonadotrophin was immunologically similar to the luteinizing principle in the Second International Reference Preparation of Human Menopausal Gonadotrophin (2nd IRP-HMG) in a double-antibody radioimmunoassay employing guinea pig antiserum to human chorionic gonadotrophin (HCG), purified human pituitary luteinizing hormone (LH)-131I as tracer and the 2nd IRP-HMG as standard. The standard curves obtained utilizing the 2nd IRP-HMG, pituitary LH and HCG were parallel in this assay. Extracts of normal liver, of liver from a patient dying of leukemia, of normal lung, kidney, spleen and muscle, and porcine insulin, human growth hormone, adi'enocorticotrophin and thyrotrophin (plasma from a child with acquired hypothyroidism) did not cross-react in this system. By radioimmunoassay the gonadotrophin content of the hepatoblastoma was 3.74 IU/g and of the pulmonary metastasis, 0.48 IU/g, while the urinary excretion of gonadotrophin was 77.7 and 104.4 IU/day on 2 occasions.