Pigs in the United States were exposed to African swine fever (ASF) virus isolated from pigs in Brazil and the Dominican Republic. The former were examined for clinical response, lesions, viremia, and antibody response. Sequential blood samples were tested for the presence of ASF virus by the hemadsorption test (in swine buffy coat cell culture) and for antibody to ASF virus by the enzyme-linked immunosorbent assay. The incubation period was 3 to 5 days; inoculated pigs had fever for 8 to 16 days (mean 12.5 days) and viremia at 3 to 35 days after inoculation and few died. Inoculated pigs developed antibodies at 7 days after inoculation which were detectable until the termination of the experiment (10th month). Reinoculation of some of the surviving pigs with the homologous isolate at approximately 6 months after exposure did not induce clinical response, viremia, nor anamnestic antibody response. In contrast, challenge exposures of convalescent pigs with the Lisbon-60 viral strain approximately 5 weeks after exposure to the Brazilian strain produced death, in spite of an anamnestic antibody response.