MATERIALS AND METHODS

Animals. Mice of inbred CBA and BALB/c strains, maintained in this department, were used throughout these experiments. In most cases CBA foetal gut was grafted into adult CBA recipients, or BALB/c foetal gut was grafted into adult BALB/c recipients (ie, as isografts). A smaller number of homografts were carried out, CBA foetal gut being grafted into BALB/c recipients. The table gives details of the numbers of isografts and homografts. Preparation of donor intestine. A mouse, judged to be about 18 days pregnant, was anaesthetised with ether; the abdomen was opened and the foetuses were removed from the uterus (the exact gestational age could be determined by histological examination of one of the foetuses). Each foetus was pinned out on a cork board, the intestinal tract was removed from the abdomen and placed in ice-cold phosphate-buffered saline (PBS) at pH 7.2. The coils of intestine were teased apart and the fragile mesentery was removed. Stomach, small intestine, caecum and colon could be easily identified morphologically. The material to be grafted (usually small intestine) was selected, transferred to another di8h of cold PBS and cut into fragments about 5 mm in length.