Because there has been no report of symptomatic Neospora caninum infection in humans, we examined the effect of human serum on the parasite's growth in either a bovine angioendothelial cell or Caco-2 cell culture in vitro and in immunocompromised mice in vivo. There was no difference in intracellular parasite numbers between cells incubated with human serum at 24 hr after challenge and those incubated with fetal bovine serum (FBS), which has no titer for the anti–N. caninum agglutination antibody test. Serum of sheep infected with N. caninum, which has the anti–N. caninum antibody, reduced the numbers of the intracellular parasite significantly. We also showed that there was no inhibitory effect on the intracellular multiplication of the parasite in cells incubated with human serum through incorporation of ³H-uracil. CB-17 scid mice administered human serum daily and challenged with N. caninum died on day 20 or 22 after challenge, when large numbers of parasite clusters were found in the brain, oviduct, adrenal gland, lung, stomach, spleen, skeletal muscle, pancreas, and mesenteric lymph nodes. Scid mice administered FBS survived until the end of the experiment. These results suggest that adult human serum may have no inhibitory effect on the development of N. caninum in vitro and in vivo.