The pathogenesis of atherosclerosis has been related to infection of the arterial wall, but it is not clear whether this occurs before or after the development of lipid-containing lesions. Respiratory bacterial infection increases the expression of vascular cell adhesion molecule-1 (VCAM-1). We therefore examined whether a similar infection would enhance atherosclerosis in New Zealand White rabbits fed chow supplemented by 15% (w/w) egg yolk for 50 days. Rabbits with naturally acquired respiratory infection by Pasteurella multocida, pathogen-free (SPF) animals infected by P. multocida in the laboratory, and age-matched SPF rabbits maintained in a disease-free environment were used. Endothelial cells expressing VCAM-1 in the aorta between intercostal arteries 3 and 5 were identified using anti-VCAM-1 (Rb1/9) and an alkaline-phosphatase-linked secondary antibody and quantified in Häutchen preparations. The remainder of the aorta was stained with Sudan IV to show lipid deposition. The expression of VCAM-1 (mean +/- SEM per 10,000 cells) was 22 +/- 8 (n = 5) in the lipid-fed SPF rabbits, significantly different from that in the lipid-fed rabbits with naturally occurring infection (190 +/- 51 (n = 5)) or from rabbits infected in the laboratory (106 +/- 25 (n = 5)). The extent of Sudanophilia was significantly greater in the naturally infected rabbits (8.3 +/- 1.2%) or infected SPF rabbits (10.3 +/- 1.8%) than in the SPF rabbits (2.7 +/- 0.8%; P < 0.05). Antibiotic treatment in naturally infected rabbits reduced the number of cells expressing VCAM-1 and the extent of the Sudanophilia to baseline levels. Thus, Sudanophilia is enhanced by bacterial infection in rabbits fed egg yolk and is associated with a significant increase in VCAM-1.