Results are presented of investigations of the running-in period in reciprocating sliding of a semicrystalline polymer, Nylon 6.6, on metal. A correlation was established between the changes in transfer film geometry and both the friction coefficient and the wear rate variations during a single stroke, as well as with sliding time. The role of wear products in running-in was found to be the determining factor. In order to avoid any unspecified influence of frictional heat variation on the sliding process, the counter specimen temperature was kept constant. Significant correlations, between both local and mean coefficients of friction and local and total areas of transfer film respectively, were found. The latter relation shows the existence of two different types of running-in process which depend on whether, initially, adhesive or abrasive wear mechanisms prevail. In both cases, the building up of the transfer film together with roll-like wear particles create several types of interactions between the rubbing bodies. These interactions, subsequently called subprocesses, have their own different specific coefficients of friction and can exist on the sliding track solely or in combination. They also change their distribution over the track and finally create a variation in the mean friction coefficient with time. This variation is expressed as a function of the total area fraction of transfer film spreading with time. It is also shown that changes in wear rate coincide with variations in mean friction coefficient. The presented model can also be extended qualitatively to other semicrystalline polymers.