The pressure drop for two-phase flow in a horizontal pipe was measured under conditions like those encountered in the suction line of refrigeration systems. At low refrigerant flow the pressure drop was found to correlate with the model recommended by Macken et al. (1979) in a previous ASHRAE project (109RP). At higher refrigerant flow the data diverged systematically from the Macken model. Further experimental study is recommended, especially needed are data at higher refrigerant flux than the present apparatus can produce and data at different pipe diameters.KEYWORDS: oils, refrigerants, mixtures, pressure drop, compressors, refrigerators, fluid flow, data, models, measuring, calculating, statistics.