This experiment examined the effects of including recombinant ovine granulocyte-macrophage colony-stimulating factor (GMCSF) in in vitro culture on secretion of interferon-tau (IFNT) by bovine blastocysts. At 32 h post-insemination (p.i.), cleaved bovine zygotes were selected and incubated with or without GMCSF for either 48 h only (between 32 and 80 h p.i., Early) or until day 9 p.i. (Throughout). Concentrations of GMCSF (ng/ml) examined were as follows: Experiment 1: 2, 5, 10 and 50 (Early only); Experiment 2: 50 (Early and Throughout); Experiment 3: 2 and 10 (Early and Throughout). In none of the experiments did GMCSF have an effect (p > 0.05) on the numbers of blastocysts formed or blastocyst characteristics as assessed by cell number, proportion of apoptotic cells or oxidation of pyruvate. When GMCSF was included in culture medium between 32 and 80 h p.i. (Early), IFNT concentrations were lower (in media drops recovered after culture of groups of embryos for 48 h between days 7 and 9 p.i. and normalized by the numbers of blastocysts developing within each drop) compared to no inclusion of GMCSF or GMCSF present Throughout culture (Experiment 2, p > 0.05; Experiment 3, p = 0.038). IFNT was present in media drops in which groups of embryos had been incubated between days 7 and 9 p.i. but in which no blastocysts had developed. Experimental treatment did not influence (p > 0.05) IFNT secretion by blastocysts incubated individually for 24 h. However, during the 24-h individual culture, blastocysts recovered on day 7 secreted less IFNT than blastocysts recovered on day 8 (mean +/- SE; 15 +/- 1.3 v 30 +/- 3.6 pg/ml; p <0.001). In conclusion, in contrast to previous studies in the ovine, GMCSF did not increase IFNT secretion but in agreement with the ovine did not affect bovine blastocyst development.