The formation of spontaneous rosettes by peripheral blood or spleen mononuclear cells when incubated with sheep red blood cells has proved a useful way of assessing the potency of immunosuppressive drugs and antilymphocyte sera in vitro. A test employing the inhibition by antilymphocyte globulin (A.L.G.) of spontaneous rosette formation around peripheral blood mononuclear cells is described. This has been used to assess the degree of immunosuppression in patients with renal allografts and uraemic patients on regular haemodialysis.Twenty-three patients with renal allografts had 21 clinically diagnosed episodes of rejection. In none of these rejection episodes was the minimal inhibitory concentration (M.I.C.) of A.L.G. (that necessary to reduce the spontaneous rosette formation of peripheral cells by 75%) less than 1/50,000. Nineteen patients had no rejection episodes during 57 patient/months of continuous observation while the M.I.C. was at a greater dilution than 1/50,000. The test has therefore been of great value in suggesting when an individual is capable of rejecting his graft, and allows the dose of immunosuppressive drugs to be adjusted to a minimum in a controlled fashion. It has been of use in diagnosing rejection in the anuric patient, when the distinction between rejection, urinary tract obstruction, and infection is particularly difficult.Fifteen patients maintained on regular haemodialysis for more than a year had, as judged by this technique, less reactive lymphocytes than normal healthy controls. The degree of immunosuppression was not as great as in the patients on full immunosuppressive regimens.