There are two ways to isolate serum from blood. One is to isolate so-called IC-serum by centrifuging immediately after drawing blood. Another is to isolate serum (called DC-serum) from blood 18 hours after clotting. IC-rat serum has been used as the most beneficial medium for in vitro culture of rat whole embryos. Rat embryos are known to develop well and normally in the IC-serum, but not with the DC-serum. In our experiments, serum was isolated from rat blood I h after clotting. This serum (3-h-DC-serum) seemed to be one of the DC-sera. Embryogenesis in whole embryo culture with 3-h-DC-serum occurred as normally as it did in the IC-serum. Development of rat embryos cultured in 50% 3-h-DC-serum was compared with that in 50% IC-serum, knowing that all embryos cultured in 30-100% IC-sera diluted with Dulbecco's Modified Eagle medium revealed normal development. As a result, it was found that the promoting effect of 3-h-DC-serum on embryogenesis was indistinguishable from that of the IC-serum. Moreover, hemolysis of rat serum within 5% was also found not to affect embryogenesis.