Intracardiac injection, in hooded Lister rats, of syngeneic MC28 sarcoma cells never induced tumour growth in normal bowel. Tumour growth occurred at the site of a colonic anastomosis if surgery preceded tumour injection but not if it followed tumour injection, even by as little as 1 h. Maximum enhancement of tumour growth occurred when the healing process had progressed between 2 and 8 days, with a peak at 5 to 7 days. The enhancing effect was largely over by the time the healing had progressed 14 days. The syngeneic OES5 breast carcinoma also grew at colonic anastomoses when surgery preceded tumour injection by 5 days, but not in normal colon. The MC28 sarcoma also grew at ileal anastomoses but not in the normal ileum after intracardiac injection. By injecting radiolabelled sarcoma cells, an estimate of the probability of a single bloodborne tumour cell lodging at a colonic anastomosis and leading to a tumour deposit was calculated to be of the order of 1:43 whereas the probability of the cell lodging in normal colon and causing a deposit is less than 1:4 x 10(4).