This study examines the efficiency, interactions and impact of a milk-run operator on a typical assembly line. The idea behind the milk-run lies in organizing the material supply to production areas using a specialized logistic worker travelling in cycles between the warehouse and production. A discrete-event simulation model was developed to evaluate the interactions of the milk-run operator and a typical 10 workstation assembly line. We present an analysis of the various kinds of disturbances occurring in the production environment (time variability of technological operations and supply cycle, delays of supply cycles) and management decisions (takt time presence or absence, buffer capacity, supply cycle duration) on the production stability and performance (assembly line throughput rate, milk-run operator utilization, workstation starvation and work in process). Recommendations for designers of in-plant logistics are provided.