This paper describes a study of the friction and metallic transfer between sliding metal surfaces in the absence and in the presence of boundary lubricant films. One surface is made radioactive and is slid over the surface of a second, non-radioactive metal, the amount of metal transferred being detected by the blackening of a photographic plate placed in contact with the second surface. The results show that, in general, the metallic transfer or 'pick-up', consists of a relatively small number of discrete particles. For unlubricated surfaces the pick-up is about 40 times larger for similar than for dissimilar metals, although the coefficient of friction covers a relatively small range ($\mu \approx $ 0$\cdot $4 to $\mu \approx $ 1). With well-lubricated surfaces the friction is reduced by a factor of not more than 20 ($\mu \approx $ 0$\cdot $05), whilst the 'pick-up' may be diminished by a factor of 20,000 or more. A simple analysis suggests that under these conditions the welded metallic junctions formed through the lubricant film play a very small part in determining the frictional resistance to motion. Consequently two lubricants possessing widely differing abilities to protect the surfaces may give essentially the same coefficient of friction. The lowest friction and 'pick-up' are observed when the lubricant film is solid. As the temperature is raised a marked increase in friction and 'pick-up' occurs at a temperature close to the melting-point of the film. A new observation is that at a somewhat higher temperature a further deterioration in lubricating properties occurs; although the surfaces are visibly covered with lubricant, the frictional behaviour and the metallic transfer are similar to those observed with unlubricated surfaces. These changes are reversible on cooling, and it is suggested that they correspond to changes in state of the lubricant film. The results provide direct support for the view that the friction between metals is due largely to the formation and shearing of metallic junctions, and that the main function of a boundary lubricant is to reduce the amount of metallic interaction. The investigation also shows that the metallic transfer is immensely more sensitive to changes in surface conditions than is the coefficient of friction.