The usefulness of supplementary dietary L-carnitine as an immunomodulator to increase antigen-specific antibody levels was analysed in 2-6-week-old broilers. The chickens received commercial feeds either unsupplemented (starter feed 17.8 mg carnitine/kg, finisher diet 22.9 mg carnitine/kg) or supplemented with L-carnitine (100 mg carnitine/kg added to feed). At 14 d of age, both groups were distributed in equal numbers and sex ratios over two environmentally controlled chambers where temperature (28 degrees) was either reduced immediately to 20 degrees, or gradually to 22 degrees at 36 d of age. Antigen-specific immunoglobulin (Ig)M, IgG, IgA and total Ig responses were measured following two immunizations with bovine serum albumin (BSA). The typical BSA-specific IgM responses followed by IgG responses to the primary immunization were boosted by the secondary immunization. The kinetics of these responses were not altered by L-carnitine treatment. However, BSA-specific total Ig and IgG, but not IgM, responses were significantly increased by dietary L-carnitine supplementation, after both the primary and the secondary immunization. No significant influence of the sex of the chicks or the imposed environmental temperature on Ig responses was found. Temperature treatment and sex, but not L-carnitine supplementation, did significantly influence body-weight gain: cockerels were heavier than females and this became most evident in the second half of the rearing period. Further, lowering the temperature increased body weight. In conclusion, dietary L-carnitine supplementation appeared to be beneficial in enhancing specific humoral responses on vaccination.