The friction properties of two groups of copper-based bearing alloys, one with nominal compositions of 6% Sn, and 0, 5, 10, 15, 20, 25% Pb, and the other with 15% Pb and 0, 3, 6, 9, 12 and 15% Sn, were studied in argon and air as well as in various lubricants of 25 ± 1 C. Without lubrication, the presence of lead is essential in that the smeared Pb film, which is formed on the bronze surface by extrusion from the matrix, protects the steel shaft from severe damage. Up to 10%, Pb also lowers the wear of the bearing material itself appreciably. Up to 9%, Sn is beneficial. When a relatively viscous lubricant is applied, a running-in period is followed by a transition towards a fully hydrodynamic condition (f < 0.01). A “critical load” (Lc) exists, below which running-in wear is negligible and above which excessive wear occurs. Under these conditions, alloys with a coarse Pb distribution behave much better than alloys with finely divided Pb. In fine-grained alloys, addition of Pb is unfavorable as Pb lowers the value of Lc, drastically. Also, this effect is explained in terms of a lubricant adsorption mechanism in which are taken into account the lubricant adsorbing function of tin and the heat conductivity of the alloys.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference held in Atlantic City, Now Jersey, October 8–10, 1968.