Four experiments were conducted to determine the composition of a solution suitable for vitrification of immature bovine oocytes. The osmotic and cytotoxic effect of different concentrations (0, 0.25, 0.5 and 1 mol l−1) of nonpermeating solutes (sucrose versus trehalose) were examined. In addition, the effect of permeating cryoprotectants such as glycerol, dimethyl sulfoxide, and propylene glycol (40% w/v) on the viability of oocytes was studied to determine the optimal time of exposure and the most suitable cryoprotectant. Exposure of bovine oocytes to trehalose was less harmful than exposure to sucrose (P < 0.01), and high normospermic fertilization (70%) was achieved after exposure to 0.25 mol trehalose l−. Propylene glycol was chosen as the cryoprotectant for the vitrification of immature bovine oocytes because of its fast permeating rate and its low cytotoxic effect. The composition of this solution (40% propylene glycol and 0.25 mol trehalose l−1 in PBS containing 4% w/v BSA) appeared to be suitable for vitrification, as the fertilization rate of the vitrified oocytes was 37% (36 of 97).