Antisera obtained from five rabbits, immunized against ovine luteinizing hormone (LH) were tested for their ability to combine with 125I-labelled ovine LH. Thirty five days after immunization commenced only one rabbit produced an antiserum which on dilution (1:20 000) could precipitate approximately 50%of the 125I-LH added. Subsequent immunization resulted in only one animal yielding an antiserum which could not be diluted at least 1:50000, while in the case of three rabbits, dilution 1:100000 was possible. The cross-reaction of a selected antiserum with an ovine plasma high in LH, thyroid stimulating hormone (TSH) and follicle stimulating hormone (FSH) was within accepted limits. Random-bred animals would appear to be suitable for the production of high-titre antisera.