Contents:The aim of this study was to determine whether the semen donor andlor heparin concentration injluences the rate of fertilization of bovine follicular oocytes and their subsequent embryonic development in vitro. Frozen-thawed semen from five highly fertile bulls was treated with one of four concentrations of heparin (0.5,l .O, 2.0 and 5.0 kglml) on a 5 x4 factorial basis in an IVM-NF programme. Zygotesloocytes were cultured in frozenthawed bovine oviduct cell-conditioned medium for 6 days. The use of semen from different bulls resulted in significantly (P < 0.001) different rates offertilization, as judged by cleavage rates of the oocytes at 72 h post insemination, and subsequent embryonic development through the '8-cell-block'(P c 0.05) in vitro. Development up to the morulalblastocyst stage, however, did not dizer significantly (P = 0.06) among groups of oocytes fertilized with spermatozoa from different bulls. Heparin levels ranging from 0.5 to 5.0 kgiml did not differ in their effect on in vitro fertilization as judged by the rate of normally cleaved ooqtes (P = 0.14). The overall parthenogenetic division rate at 72 hpost insemination was 12.4% andwas not influenced by the heparin concentration. There was a linear relationship (P < 0.001) between fertility estimates based onAIand the estimates basedon thefirst cleavage following in vitro fertilization.