Two experiments were conducted to compare the feedlot performance of swine fed diets containing a commercially available lactobacillus probiotic and virginiamycin, a gram-positive antibiotic (experiment I) and to evaluate the feedlot performance of starter and grower-finisher swine housed under conditions of restricted and adequate space allowance fed diets with and without virginiamycin (experiment II). For experiment I, in four starter trials, pigs fed diets containing virginiamycin tended to eat more and grow faster than pigs fed the control diet while lactobacillus probiotic had no effect on performance. In the combined analysis of a starter-grower-finisher and a grower-finisher trial, virginiamycin supplementation had no effect on performance while the pigs fed the probiotic had significantly poorer gains than the control pigs. In three grower-finisher trials, virginiamycin supplementation improved daily gain and feed consumption while lactobacillus probiotic had no significant effect on performance. For experiment II, in four starter trials, increasing the number of pigs in 1.2 x 1.2 m nursery pens from six to 12 caused significant depressions in final weight, daily gain, feed intake, and feed efficiency. Virginiamycin supplementation significantly improved final weight, daily gain and feed efficiency. In two starter trials, reducing space allowance from .24 to .12 m² per pig caused significant depressions in final weight, daily gain and feed efficiency but virginiamycin had no effect on performance. In a series of grower-finisher trials, decreasing space allowance from .78 to .43 m² pig caused significant depressions in final weight, daily gain, feed intake and feed efficiency while virginiamycin improved feed efficiency. The virginiamycin X space allowance interaction was significant in only one instance with virginiamycin improving feed efficiency 6.2% when starter pigs were given adequate space allowance but only 2.5% when crowded. These findings suggest that virginiamycin is superior to the probiotic as a growth promotant for swine. Also, housing pigs under crowded conditions does not increase the level of response to virginiamycin. / Master of Science