In order to improve the culture of oocytes that remain in an immature stage, the protein content before maturation and developmental ability of different groups of oocytes were investigated. The oocytes were classified according to source (adult or calf), the size of the follicle and the cumulus aspect. The influence of cumulus cells on developmental capacities was also analyzed and their protein patterns differ from those obtained with oocytes. The protein patterns differ specifically at 140 kDa and also at 143, 137, 60, 58, 52 and 27 kDa between oocytes from different sources, as does their capacity to develop into morula. Following in vitro maturation and fertilization, the oocytes were evaluated for percentage embryo development to the morula stage after 5 d. For cows' ovaries, denuded oocytes and oocytes with a defective cumulus had a percentage development of 2 and 12%, respectively. Cow ovary oocytes that were selected for cumulus compactness of 1-3 mm and > 3 mm had a percentage development of 21 and 25%, respectively. Oocytes from calf follicles had 0% development. These results indicate that the source of oocyte is a major determinant of its developmental capacity.