Body growth, feedlot performance and carcass characteristics of ram lambs (n = 16) immunized against luteinizing hormone-releasing hormone (LHRH) at 10 weeks of age with recombinant LHRH fusion proteins were investigated. Recombinant fusion proteins, ovalbumin-LHRH-7 and thioredoxin-LHRH-7 were produced using recombinant DNA technology. Animals were immunized (n = 8) against LHRH with ovalbumin-LHRH-7 and thioredoxin-LHRH-7 recombinant protein mixture in the Freund's complete adjuvant. The immunization group received two booster injections 4 and 8 weeks later, with Freund's incomplete adjuvant. Animals in control group (n = 8) were not treated. Animals were kept at relatively poor pasture until 27 weeks of age. This was followed by a 70 days finishing period that involved housing in groups and ad libitum feeding with concentrate. Carcasses were evaluated after chilling for 24 h at +4 degreesC. Live weights, finishing weight, weight gain and average daily weight gain were similar between groups (P > 0.05). Carcass measurements, loin eye muscle area and back fat thickness were not affected from immunization. Immunization did not affect hot and cold carcass weights, dressing percentage, offal items and wholesale cuts weights. Immunized animals had smaller testis, chop and bone weights than control animals (P < 0.05). It was concluded that immunological castration could be achieved at 10 weeks of age in ram lambs using new recombinant LHRH fusion proteins and used in finishing programs without adverse effect on growth rate, feedlot performance and carcass characteristics. Nevertheless, the effectiveness of these proteins should be further evaluated with more animals. (C) 2003 Elsevier B.V. All rights reserved.