Objective: Ureaplasma spp are the most commonly isolated microorganisms in association with preterm birth. Maternal erythromycin administration is a standard treatment for preterm prelabor rupture of membranes. There is little evidence of its effectiveness in eradicating Ureaplasma spp from the intrauterine cavity and fetus. We used a sheep model of intrauterine Ureaplasma spp infection to investigate the efficacy of repeated maternal intramuscular and intraamniotic erythromycin treatment to eradicate such an infection. Study Design: Thirty ewes with singleton pregnancies received an intraamniotic injection of 107 color change units of erythromycin-sensitive Ureaplasma parvum serovar 3 at 55 days' gestation. At 116 days' gestation, 28 ewes with viable fetuses were randomized to receive (1) intraamniotic and maternal intramuscular saline solution treatment (n = 8), (2) single intraamniotic and repeated maternal intramuscular erythromycin treatment (n = 10), or (3) single maternal intramuscular and repeated intraamniotic erythromycin treatment (n = 10). Fetuses were surgically delivered at 125 days' gestation. Treatment efficacy was assessed by culture, quantitative polymerase chain reaction, and histopathologic evaluation. Results: Animals treated with intraamniotic erythromycin had significantly less viable U parvum serovar 3 in the amniotic fluid at delivery. However, neither combination of maternal intramuscular and intraamniotic erythromycin treatment successfully cleared U parvum serovar 3 from the amniotic fluid or fetal tissues. Three de novo erythromycin-resistant U parvum isolates were identified in erythromycin-treated animals. Conclusion: Erythromycin treatment, given both to the ewe and into the amniotic cavity, fails to eradicate intrauterine and fetal U parvum serovar 3 infection and may lead to development of erythromycin resistant U parvum.