We have recently shown in a rat model that the acute decrease in glomerular filtration rate caused by intravenous Sandimmun can be avoided if Cyclosporine A (CyA) is given in a fat emulsion carrier instead. The present study was undertaken to determine the immunosuppressive effect in vitro of this new formulation. The inhibitory effect on the lymphocyte responsiveness of CyA in fat emulsion carrier was compared with that of CyA dissolved in alcohol, using mitogens as well as alloantigens as stimuli. In the mixed lymphocyte culture and cell-mediated lympholysis tests and in the interleukin-2 assay, both formulations seemed equally effective. CyA in fat emulsion was somewhat less effective than CyA in alcohol in the mitogen tests. In clinical practice the fat emulsion would be rapidly metabolized, and to further evaluate our new formulation, studies in experimental organ transplantation are needed.