SUMMARY: Porcine fetuses were inoculated with SMEDI C virus (porcine enterovirus strain PS34) at 3 stages of development approximating 2, 4, and 6 weeks before end of gestation (68 to 70, 82 to 84, and 96 to 98 days of gestation). Inoculation was made in utero, utilizing surgical procedures. Hysterectomy of sow was done at different postinoculation (pi) periods to determine lethality of the virus to fetus, fetus-to-fetus transmission, and areas of greatest viral concentration in fetal tissues, and to detect virus by fluorescent antibody technique (fat) or tissue culture isolation. The fetal immune response was determined with a cell culture-serum neutralization system. It was concluded that inoculation of SMEDI C virus (PS34) caused death when the fetus was infected during the later part of gestation. Intrauterine transmission took place, as evidenced by death and by the presence of antibody in noninoculated fetuses. The highest concentration of virus was in lung and intestine in the fetus regardless of the age when inoculated. When virus was present in high concentrations, it was readily detected by fat. Antibody levels in contact-exposed fetuses, though not always detectable, subsequently were equivalent to those in inoculated fetuses. Immunologic competence of the porcine fetus against porcine enterovirus began to develop near 68 days of gestation and I reached maturity between 84 and 96 days of gestation.