The effect of passively acquired antibodies on the course of African swine fever (ASF) virus infection was investigated in hysterotomy-derived neonatal pigs fed colostrum from an ASF-recovered sow or given ASF virus antiserum. Thirty neonatal pigs were assigned to 5 study groups: (i) colostrum-deprived, (ii) fed colostrum from a normal sow, (iii) fed colostrum from an ASF-recovered sow, (iv) given ASF virus antiserum, and (v) noninoculated controls. Pigs were inoculated oronasally with 10(6.1) median hemadsorption units (HAd50) of a Dominican Republic ASF virus isolate. The progression of ASF infection was monitored by measure of rectal temperature, viremia titers, antibody response, and observation of attitude. The clinical course of ASF infection was markedly different in young pigs in the various study groups. On postinoculation day (PID) 4, ASF viremia titers for pigs receiving colostrum from an ASF-recovered sow or ASF virus antiserum (mean = 3.2 +/- 1.88 log10 HAd50, n = 10 pigs) were significantly lower (P < 0.001) than viremia titers of colostrum-deprived pigs or of those fed normal colostrum (mean viremia titer = 7.8 +/- 0.55 log10 HAd50, n = 14 pigs). All pigs not given colostrum or serum (n = 15 pigs) from swine recovered from ASF were dead by PID 16. By PID 30, only 1 pig that received colostrum or antiserum (n = 10 pigs) from the sow recovered from ASF had died. To determine whether the protective effect of ASF antiserum resided within the immunoglobulin (Ig) fraction, 4 pigs that had acted as noninoculated controls for the 1st experiment were given 125 mg of ammonium sulfate precipitated Ig from the ASF virus antiserum used in the initial study (intraperitoneally). The 5th pig was not given Ig (nontreated-inoculated control). All 5 pigs were inoculated oronasally with 10(6.1) HAd50 of Dominican Republic ASF virus. The nontreated control pig died on PID 10 and 3 Ig-treated pigs died on PID 17, 23, and 24. The 4th Ig-treated pig survived. Although administration of precipitated ASF Ig did not completely protect against clinical ASF infection or death, the course of infection was markedly altered.