Thirty-three Holstein cows were superovulated with follicle stimulating hormone. Embryos were non-surgically recovered 6 days post-mating. Ninety embryos were morphologically evaluated and placed in culture in Hams F-10 media supplemented with a 10% protein source (steer serum, bovine serum albumin, or uterine flushings). Embryo development was observed every 12 h. Treatment differences were evaluated by assigning a numerical value of Oto 4 to each embryo representing the most advanced stage of development reached in vitro (O=no development, 4= hatched blastocyst). Data were then analyzed by analysis of variance, and Chi-square contingency analysis. Development of embryos was comparable in steer serum and bovine serum albumin but differed significantly from heat-treated or non heat-treated uterine flushings. Since albumin alone was able to support embryo growth it suggests that the albumin component of steer serum may be responsible for the development observed with that treatment. However, the possibility of non-albumin compounds contributing to the success of this protein source cannot be ruled out. Uterine fluids were unable to promote growth of embryos in vitro. Several explanations can be offered. The uterine fluids utilized were collected from ovariectomized hormonally induced cows and may not reflect the naturally occurring proteins of the normal uterus. The proteins may reflect a uterine stage which does not coincide with the age of the embryos. The level at which the uterine proteins were supplemented may have exceeded normal levels in the uterus and may have become inhibitory and lastly, the uterine fluids may have contained an abnormal amount of immunoglobulins which could interfere with normal embryo development. Analysis of the mean time to reach each stage demonstrated the similarity in the time course of development between the embryos supplemented with steer serum or bovine serum albumin in culture and embryos in vivo. This suggests that culture conditions other than uterine flushing supplementation were very favorable to embryo development. / M.S.