There are significant advantages to the production of gene-knockout mice directly in mouse strains other than 129. The availability now of ES cells derived from the C57BL/6 mouse strain presents workers with a valuable alternative. A major difficulty, however, is the requirement for BALB/c blastocysts as recipients for ES cell injection. Using standard procedures, few BALB/c blastocysts can be obtained. This limitation has now been resolved by harvesting BALB/c embryos at the early morula stage and maturing these to blastocysts by in vitro culture. Of early morulae harvested and cultured, over 70% were recovered as fully expanded and injectable blastocysts. C57BL/6 ES cell injection of these blastocysts has enabled the production of a number of gene-knockout mice with a success rate similar to that reported for ES cells derived from the 129 mouse strains.