PORCINE REPRODUCTIVE AND RESPIRATORY VIRUS (PRRSV) CROSSES THE PORCINE PLACENTA AND CAUSES A COMBINATION OF FETAL DEATH, AND DEVELOPMENTAL DAMAGE. ALL TOLD, THE VIRUS IS ESTIMATED TO COSTS THE US SWINE INDUSTRY $1.2B PER YEAR, OF WHICH $380M IS ASSOCIATED WITH ITS CONSEQUENCES IN THE BREEDING HERD. PREVENTING VERTICAL TRANSMISSION OF THE VIRUS FROM THE MOTHER IS THE IDEAL WAY TO ELIMINATE THESE EFFECTS, BUT THE MECHANISM BY WHICH THE VIRUS CROSSES THE PIG'S PLACENTA HAD NOT BEEN ESTABLISHED. THE LEVEL OF FETAL INFECTION VARIES WITHIN A LITTER OF PIGS, AND WE HYPOTHESIZE THAT TRANSMISSION OF THE VIRUS IS DEPENDENT ON THE SPECIFIC CHARACTERISTICS OF ANY GIVEN FETUS'S PLACENTA. OUR OBJECTIVE IS TO EVALUATE THE IMPACT OF GESTATIONAL AGE AND THE DEGREE OF INTRAUTERINE GROWTH RETARDATIONS ON THE LEVEL OF FETAL INFECTION WITHIN A LITTER. WE EXPECT TO SHOW THAT THE MID GESTATION AND GROWTH RESTRICTED FETUS IS NATURALLY RESISTANT TO PRRSV, AND WILL USE THIS INFORMATION TO IDENTIFY THE UNDERLYING MECHANISM BY WHICH PRRSV CROSSES THE PLACENTA. INFORMATION IN THIS AREA WILL BE CRITICAL IN DEVELOPING STRATEGIES TO REDUCE OR ELIMINATE FETAL PRRSV INFECTION, AND THEREBY BY INCREASE PROFITABILITY WITHIN THE SWINE HERD.