Mullerian inhibiting substance (MIS) is a testicular hormone made by fetal and postnatal Sertoli cells, and causes Mullerian duct regression during male sexual dilferentiauon. We established an EIA to measure MIS scrum levels in a number of physiological and pathological conditions. Significant levels of serum MIS are found in normal males during the first year of life (1). MIS levels decline throughout childhood and are undeteclable after pubertal development (2). MIS levels were examined in patients previously diagnosed with Hodgkin's Lymphoma. Nine adults (>16 years) had low bui measurable levels of MIS. Patients with Hodgkin's Lymphoma have azospermia, suggesting a possible link between MIS secretion and the spermalogonia. In very low birth weight babies (>1500g). MIS levels in cord sera were significantly higher than in full-term controls (79125μg/L vs 36±8μg/L; P<0.001). MIS levels also were examined previously in infants and boys with cryptorchidism, where the levels during the first year were significantly lower than normal (3). This suggested a possible link between MIS and germ cell maturation. which is occurring at that time. Recently we have confirmed experimentally that MIS does stimulate germ cell maturation in the neonatal mouse testls in vitro. (1) Baker ML, Metcalfe SM & Hutson JM. J Clin Endocrin Metab. 1990; 70: 11-15. (2) Baker ML & Hulson JM. J Clin Endocrin Metab (In Press) (3) Yamanaka J, Baker ML, Metcalfe SM & Hutson JM. J Pediatr Surg. 1991; 26: 621-623