The density of the corona radiata as a marker for the quality of cumulus-corona-oocyte complexes (CCOC's) for in vitro embryo production was tested. The CCOC's in which the corona radiata appeared as a dark rim surrounding the zona pellucida (Group 1) and CCOC's in which the corona had the same density as the rest of the cumulus investment (Group 2) were assessed with respect to nuclear ultrastructure, corona-cumulus expansion and capacity for sustaining embryonic development in vitro. An intermediate Group 3 with characteristics between Groups 1 and 2 was also assessed for in vitro development capacity. The CCOC's in Group 1 were typically meiotically unactivated and presented a nonundulating nuclear envelope. More than half of the CCOC's in Group 2 showed some degree of meiotic activation, and those that were nonactivated displayed ''holes'' in the nuclear envelope or dilatations of the perinuclear cisterna. The CCOC's in Group 2 were characterized by partial corona-cumulus expansion already at collection. The CCOC's from Groups 1 and 3 sustained embryonic development in vitro at a significantly higher rate than CCOC's from Group 2. It is concluded that CCOC's in which the corona radiata has the same density as the rest of the cumulus investment are less competent candidates for in vitro embryo production.