A recombinant ovalbumin-luteinizing hormone-releasing hormone (ova-LHRH) antigen has been developed for immunocontraception. In this study, a novel immunostimulant for ova-LHRH immunization, CpG oligodeoxynucleotide (ODN) 2006, was compared against Mycobacterium butyricum. Also, the immunogenicity of ova-LHRH after lyophilization and exposure to organic solvents was assessed. Rats received either ova-LHRH solubilized in urea; lyophilized ova-LHRH; lyophilized ova-LHRH exposed to methylene chloride; or lyophilized ova-LHRH exposed to ethyl acetate. Immunogenicity of lyophilized ova-LHRH was reduced compared with solubilized ova-LHRH. Exposure to ethyl acetate further decreased immunogenicity of ova-LHRH. CpG ODN 2006 was a more effective immunostimulant than M. butyricum for LHRH immunization. (C) 2007 Elsevier Ltd. All rights reserved.