Tissues obtained from pigs inoculated with African swine fever virus (ASFV), fixed by vascular perfusion using glutaraldehyde, and embedded in paraffin or araldite were used for an immunohistologic electron microscopic study. To detect ASFV antigens, 4 methods were used on paraffin sections with or without pretreatment of the tissues. Use of biotinylated anti-ASFV antiserum combined with avidin-biotin complex and peroxidase proved to be the most suitable method, and antigen was detected in tissues infected with 2 ASF viruses of different virulence. Use of the glutaraldehyde fixation method should ensure optimal morphologic (structural and ultrastructural) data while allowing an immunohistologic study, and add to knowledge of the pathogenesis of ASF.