We have purified factor VIII/von Willebrand factor complex from rabbit plasma and prepared a specific guinea-pig antiserum to the von Willebrand factor antigen. It was used for electroimmunodiffusion to determine von Willebrand factor level in rabbit plasma and for crossed immunoelectrophoresis to study its electrophoretic behavior. Rabbit von Willebrand factor had properties similar to human von Willebrand factor with respect to molecular weight, size heterogeneity, mobility and pattern in electroimmunodiffusion and crossed immunoelectrophoresis, and content in plasma. Plasma von Willebrand factor antigen levels in rabbits ranged between 46 and 147% of the standard plasma. Responses of plasma von Willebrand factor antigen and factor VIII to various bioactive agents were investigated with rabbits.When a small dose of adrenalin, a large dose of noradrenalin, and endotoxin were injected, plasma von Willebrand factor antigen level and ratio of von Willebrand factor antigen to factor VIII activity were elevated in different fashions during 6 hours after administration.The findings could deliver some information about the mechanism of changes in plasma von Willebrand factor observed in human diseases.