Insulin-like peptide 3 (INSL3) is a peptide hormone belonging to the insulin/relaxin superfamily, which mediates testes descent in the male fetus, and suppresses male germ cell apoptosis and promotes oocyte maturation in adults by activating the leucine-rich repeat-containing G-protein coupled receptor RXFP2. In a previous work, we prepared mature two-chain INSL3 by recombinant expression of a designed single-chain precursor in Escherichia coli and subsequent in vitro maturation. To establish a convenient high throughput receptor-binding assay for screening novel RXFP2 agonists or antagonists, in the present study we designed and recombinantly prepared a fully active easily-labeled INSL3 analog. Due to presence of a single primary amine moiety, the easily-labeled analog was conveniently mono-labeled by a DTPA/Eu3+-moiety at the A-chain N-terminus through reacting with excess modification reagent in a simple one-step procedure. The DTPA/Eu3+-labeled INSL3 analog bound receptor RXFP2 with high affinity and low non-specific binding. Using this non-radioactive tracer, we established a high throughput cell-based receptor-binding assay for screening of novel RXFP2 agonists or antagonists in future studies. (C) 2012 Elsevier Ltd. All rights reserved.