Ram lamb epididymitis, a serious reproductive disease of sheep, is caused principally by bacteria belonging to the genera Haemophilus and Actinobacillus. Six bacteria were studied: the American Type Culture Collection (ATCC) of Actinobacillus seminis (ATCC 15768), ATCC of Actinobacillus actinomycetemcomitans (ATCC 29522), field isolates of A seminis (86722 and 4101) and field isolates of Actin obacillus- like bacteria (Y136 and D107). The objectives of this study were to quantitate the adhesion of these 6 bacteria to bovine kidney epithelial cells (BKECs) and ram epididymal epithelial cells (REECs), evaluate the effect of rabbit polyclonal antibody prepared against ATCC 15768 (PoAb 15768) on bacterial adherence to BKECs and REECs, and partially characterize the adhesins present on these bacteria. In a bacterial adhesion assay (BAA), strain and species differences were noted. The number of bacteria adhering to each BKEC ranged (rom a low of 4.27 ± 1.00 (Actinobacillus-like 0107) to a high of 31.84 ± 2.00 (A seminis 86722). The number of bacteria adhering to each REEC ranged from a low of 3.05 ± 0.34 (Actinobacill us-like 0107) to a high of 21.61 ± 2.03 (Actinobacillus like Yl36). In a bacterial inhibition assay (BIA), PoAb 15768 inhibited the adhesion of ATCC 15768 to both BKECs and REECs by 5%. This same antiserum inhibited the adhesion of ATCC 29522 to BKECs by 14.5% and to REECs by 22%. The inhibition of A seminis 86722 adherence to BKECs and to REECs was less than 14% and 35%, respectively. For A seminis 4101, Actinobacillus-like Y136, and Actinobacillus-like 0107, PoAb 15768 failed to prevent adhesion to either BKECs or REECs. When the 6 bacteria were analyzed by autoradiography, 2 (Actinobacillus-like 0107) to 8 (ATCC 29522) potential adhesins were identified. However, the pathogenicity has not been firmly established for many Actinobacillus species and Actinobacillus-like bacteria. The potential adhesins identified in this study were not unequivocally confirmed as bacterial adhesins. An in vit ro model may facilitate the recognition of potential adhesins used by Actinobacillus species and Actinobacillus-like bacteria and may eventually lead to the development of an efficacious bacterin to prevent epididymitis in ram lambs at risk.