Fifteen seminal characteristics were measured in ejaculates from 4 laboratory stallions and from 164 commercial stud stallions. Complete field and laboratory data were available from 536 and 531 ejaculates, respectively. These were obtained over 4 breeding seasons (1974/75-1977/78) and 9 breeds were represented. Stallions at commercial studs produced 1-13 ejaculates at intervals of approximately 4 weeks and ranging from 1 h to 1 year apart. Intra-class correlations or 'repeatability' of each seminal characteristic were calculated. Significant between-stallion variation occurred in all characteristics except for pH of gel-free semen. Low repeatabilities (less than 0.3) were estimated for gel-free volume, pH of gel-free semen and the percentage of abnormal mid-pieces. Moderate repeatabilities (0.3 to 0.6) occurred for total volume, gel volume, motility, percentage of dead spermatozoa, sperm concentration, total sperm number and the percentage of sperm head abnormalities, tail abnormalities, loose heads and cytoplasmic droplets. It is suggested that if a seminal examination shows characteristics below or near recommended minimum levels, an additional 1 or 2 samples should be examined before a stallion is classed as likely to have doubtful fertility.