Semen samples were collected from 21 Danish Warmblood stallions by the Colorado artificial vagina (Colorado AV, 14 samples) or by the Missouri artificial vagina (Missouri AV, 7 samples). The semen was examined bacteriologically by direct plating (DP) on blood agar plates, and by plating of semen swabs stored in Stuart's transport media (TM) at 4 degrees C for 1-4 days. No significant differences were observed between results obtained by DP and cultures of identical TM samples. Of the 21 samples examined, only 1 TM (4.8%) and 2 DP samples (9.5%) were sterile, while the rest yielded a predominantly mixed flora comprising 1 to 4 bacterial genera. The natural flora was dominated by coagulase-negative staphylococci (Staphylococcus lentus, S. capitis, S. haemolyticus, S. xylosus) (16/21 = 76%), coryneforms (11/21 = 52%) and alpha-hemolytic streptococci and lactobacilli (7/21 = 33%). Potential venereal pathogens were isolated from 7 stallions (33%). Beta-hemolytic streptococci were found in 4 stallions used for natural service, whereas Pseudomonas aeruginosa serotype 6 (2 samples) and Klebsiella pneumoniae subsp. pneumoniae capsule type K5 (1 sample) were isolated from 3 stallions used exclusively for artificial insemination. The role of the stallion as a carrier of potential venereal pathogens, and the artificial vagina as a source of contamination, is discussed in the context of mare endometritis.