Mice were induced to superovulate and 2-cell embryos were cultured in Whitten's medium with 10 mg bovine serum albumin/ml (WM) as control, Medium WM with 2.3, 4.6, 23.1 or 46.2 micrograms plasmin/ml, Medium WM with 14.6, 29.1 or 145.7 micrograms plasminogen/ml, Medium WM with 0.1, 0.2, 1.1 or 2.2 micrograms trypsin/ml; Medium WM with 0.2, 0.3, 1.6 or 3.3 micrograms pronase/ml and Medium WM with 10% heat-treated bovine serum (HTBS). Proteolytic activities in the culture media were evaluated at the start of the culture period and 10 days later. Blastocyst formation was significantly reduced in cultures supplemented with pronase and in the two higher levels of trypsin when compared to that in Medium WM. More embryos developed to the blastocyst stage in Medium WM + 2.3 or 23.1 micrograms plasmin/ml and Medium WM + 14.6 micrograms plasminogen/ml than in Medium WM (P less than 0.05). The incidence of hatching was significantly greater in Medium WM than in all plasminogen- and plasmin-supplemented media except for Medium WM + 29.1 micrograms plasminogen/ml. Although not significantly different, hatching was lower in Medium WM and Medium WM + 0.1 microgram trypsin/ml when compared to Medium WM + HTBS. Similar numbers of embryos completed the hatching process in Media WM, WM + 0.1 or 0.2 micrograms trypsin/ml and WM + 0.3 micrograms pronase/ml. Since dissolution of the zona pellucida occurred within 96 h for embryos cultured in Media WM + 1.6 or 3.3 micrograms pronase/ml and WM + 1.1 or 2.2 micrograms trypsin/ml, hatching could not be evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)