The pressure drop and void fraction of two-phase helium in a horizontal flow loop driven by a single stroke bellows pump at mass flow rates ranging from 0.5 to 2.0 g s-' and under system pressures between 0.65 and 1.2 atm are reported. The slip ratio (the ratio of the velocity of the gaseous phase over that of the liquid phase) in two-phase helium is deduced from void fraction data obtained with an in-line capacitance probe. The slip ratio is found to be always greater than unity and to decrease towards unity with increasing mass flow rate and increasing system pressure. Results for the twophase pressure drop measurements are analysed in terms of the conventional twophase pressure drop multiplier, which is found to depend on system pressure and vapour quality. The mass flow rate does not appear to have an effect on the two-phase frictional multiplier. A slight tube size effect on the two-phase pressure drop multiplier is found at 1.2 atm; however, this effect quickly diminishes as the system pressure reduces. In aeneral. for the ranae of oarameters studied the homogeneous model gives a much better prediction -of the two-phase friction multiplier-than does the Lockhart-Martinelli correlation.