Measurements of liquid film thickness, liquid-to-wall shear stress, pressure drop and visual observations have been carried out in a 50.8 mm i.d. horizontal pipe flow loop. Attention was paid to the lateral variation of the liquid properties by making measurements at location THETA = 0-degree (pipe bottom) and THETA = 45-degrees Statistical analysis of liquid film records led to the determination of local mean thickness, RMS values as well as of other wave characteristics (wave amplitude, intermittency, etc.) useful in computing gas-liquid interface friction. These data were complemented by similar statistical information from the shear stress measurements. The gas-liquid interface was found to deviate significantly from the usually assumed flat profile. A new result obtained here is that a small film thickness is associated with reduced local shear stress; i.e. that a lateral mean shear stress variation exists in this flow regime. In general, larger intensities of wall stress fluctuations are measured in thin films; e.g. at THETA = 45-degrees as compared to THETA = 0-degree (thicker film). Power spectra of film thickness and of shear stress display similarities indicative of the effect of waves on wall stress. There is also evidence suggesting that damping of interfacial waves takes place in relatively thick films. Probability densities of local layer thickness and of wall stress exhibit striking similarities indicative of a substrate on which a moving wavy layer develops. Using the above new data, improved estimates of the interfacial friction factor f(i) are obtained. The data exhibit a linear dependence of f(i) on the liquid layer Reynolds number, as in previous studies. An expression relating an equivalent interface roughness to wave characteristics is also presented.