To determine the effect of different levels of gonadotropin-releasing hormone (GnRH) antibody titres (AT) on plasma hormone concentrations, behaviour, testes size and performance, Friesian bull calves (no. = 72) were immunized against either human serum albumin (HSA) conjugated to Cys-Gly-GnRH (no. = 48; immunized) or HSA (no. = 24; controls) at 8 to 10 weeks of age. One booster immunization was administered on either day 28 or 56 (no. = 24 GnRH-immunized and no. = 12 control per booster day). Based on AT 1 week post respective booster, 12 immunized and six control bulls were allocated to one of three AT groups, control (C), medium (M) and high (H) with mean AT of 0·3 (s.e. 0·1), 32 (s.e. 2) and 51 (s.e. 2)% binding at a plasma dilution of 1: 160; respectively. Bulls were blood sampled and weighed every 14 days, and testicular measurements taken every 28 days, from days 0 to 533. Behaviour was observed for 4 h once weekly while bulls were together at pasture and, within AT group, for 20 min 1 week before slaughter. At slaughter, testes and carcass measurements were made. Appropriate data were analysed by ANOVA and correlation coefficients (i) determined. Behavioural data were analysed using x2. Mean AT for M and H treatment groups during the experiment were 36·5 and 44·8% binding at a 1: 160 dilution (P > 0·05, pooled s.e.d. 5·9%), respectively. Level of AT decreased (P < 0·05) mean testosterone concentrations (1·74, 1·36 and 1·21 (s.e.d. 0·17) ng/ml for the C, M and H treatment groups, respectively) and testes size (e.g. length, 10·0, 8·9 and 8·1 (s.e.d. OS) cm) in the M and H groups compared with the C bulls. There was no effect of AT level on luteinizing hormone and insulin-like growth factor-1 concentrations or on average daily gain. Bulls in the M group had the lightest testes and lowest masculinity score compared with the C and H groups (both P < 0·05). Bulls in the H group had greater backfat thickness than the C bulls (P < 0·05). Bulls in the M and H groups were less aggressive and sexually active than the C bulls (P < 0·05). In conclusion, GnRH AT in both the M and H groups persisted for the duration of the experiment, and these AT were significantly different for a critical period pre-puberty; however they both had similar effects and caused extended periods of reduced testes growth, testosterone concentrations, and aggressive and sexual behaviour compared with C bulls but there was no detrimental effect of AT on growth or carcass characteristics.