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A plasticity index of the order of 0.6 to 1 has been thought to indicate the beginning of plastic flow of asperities during contact, and is assumed to be the point of initiation of scuffing failure. This study shows that an extensive amount of plastic flow is involved in scuffing failure and the load carrying capacity is dependent not only on the amount of plastic flow but also on the adsorbed films protecting the sliding surfaces from the metallic contact. In a low-speed single-pass lubricated sliding contact, the load carrying capacity is inversely proportional to the slope of asperities for a combination of surface metal and lubricant which do not react with each other; the opposite is observed for a reactive combination of metal and lubricant. In repeated sliding, an unreactive combination of metal and lubricant carries low load to scuffing failure regardless of the slope of asperities; a reactive combination of metal and lubricant shows an inversely proportional relation between the scuffing load and the instantaneous slope of asperities measured before scuffing. Very smooth surfaces are very prone to scuff, regardless of reactivity between the metal and lubricant.