Mongrel dogs were given either whole blood (50 ml) or platelets prepared from 50 ml blood intravenously on three occasions before transplantation of a kidney from a different donor to that of the blood or platelets. All dogs were given azathioprine and prednisolone after transplantation. 60 X 10(8) platelets were obtained from 50 ml blood and the leucocyte contamination was less than one leucocyte per 10(5) platelets. The mean survival of kidney allografts in non-transfused dogs was 11.6 days (10 dogs), in dogs pretreated with whole blood 26.6 days (5 dogs), and in dogs pretreated with platelets 29.2 days (5 dogs). Sensitisation occurred in 3 of 5 dogs given whole blood and in 2 of 5 dogs given platelets. Thus pretreatment with a relatively pure preparation of platelets will produce prolongation of survival of third party renal allografts in mongrel dogs given azathioprine and prednisolone, comparable to that produced by whole blood. However, sensitisation was produced by the platelet preparation, presumably due to the minimal leucocyte contamination, which might also be responsible for the suppressive effect of this platelet pretreatment protocol.