Canine leishmaniasis caused by Leishmania infantum is endemic in the foxhound population in North America. Studies of canine leishmaniasis in the Mediterranean basin indicate a role for both CD4+ and CD8+ lymphocytes with clinical illness and in asymptomatic dogs. Limited information is available on the strain of L. infantum infecting foxhounds in North America. The present study investigated changes in cellular immune responses in dogs experimentally infected with 1x10(7) (low dose, LD; N=4) or 2x10(8) (high dose, HD; N=4) promastigotes of a United States isolate of L. infantum and control dogs (N=2) for 72 weeks. Density gradient separation was used to enrich for peripheral blood lymphocytes from canine blood. Lymphocyte subsets (CD4+ and CD8+) were quantified by flow cytometric analysis. Lymphocyte population expression levels over the course of the present study were compared to clinical status of the dog and antibody responses in infected and control dogs. No significant differences (P>0.05) were observed in either CD4+ or CD8+ lymphocyte expression in of the groups over the experimental period. This study suggests that the cellular immune responses to North American L. infantum in experimentally infected dogs may differ from other strains of L. infantum.