ACE-031 is a soluble form of activin receptor type IIB (ActRIIB). ACE-031 promotes muscle growth by binding to myostatin and other negative regulators of muscle mass. Methods: This double-blind, placebo-controlled study evaluated the safety,pharmacokinetics, and pharmacodynamics of ACE-031 in 48 healthy, postmenopausal women randomized to receive 1 dose of ACE-031 (0.02-3 mg/kg s.c.) or placebo (3:1). Results: ACE-031 was generally well-tolerated. Adverse events included injection site erythema. Mean ACE-031 AUC(0-∞) and C(max) increased linearly with dose; mean T(½) was 10-15 days. Statistically significant increases in mean total body lean mass (3.3%; P = 0.03, by DXA) and thigh muscle volume (5.1%; P = 0.03, by MRI) were observed at day 29 in the 3 mg/kg group. Statistically significant changes in serum biomarkers suggest ACE-031 also improved bone and fat metabolism. Conclusions: Single-dose ACE-031 treatment was generally well-tolerated and resulted in increases in muscle mass in healthy postmenopausal women.