The compatibility of cyclosporine with fat emulsion was studied. Admixtures consisted of cyclosporine injection at final drug concentrations of 0.5 and 2.0 mg/mL in 10% and 20% fat emulsion. Fat emulsion stability was determined by visual inspection, pH testing, and particle-size measurements at 0, 24, and 48 hours. Cyclosporine stability was measured with high-performance liquid chromatography at the same intervals. The admixtures were stored in glass containers at 23-25 degrees C under fluorescent light. Mean particle diameters remained relatively constant in all the admixtures throughout the study period. No particles larger than 6 microns were observed. Cyclosporine retained more than 96% of its initial concentration in all the admixtures. There were no changes in color or consistency, and pH did not vary by more than 0.11 pH unit. Cyclosporine 0.5 or 2 mg/mL was compatible with 10% or 20% fat emulsion at room temperature in glass containers for up to 48 hours.