The MPEG format is becoming attractive for the distribution of audio and video over the Internet because of its wide adoption. Nevertheless, the quality perceived by a user of a video transmission may be strongly degraded by indiscriminate discards of packets under the best-effort model. The Differentiated Services (DS) architecture proposes to overcome the limitations for quality of service (QoS) provisioning of today's best-effort Internet. This paper provides a quantitative study and assessment through simulation on how a DS mechanism can improve the delivery quality of MPEG video streams. We use a drop differentiation mechanism to avoid the error propagation effect of a dropped packet in the hierarchical structure of frames in MPEG systems. Results show that the drop differentiation mechanism improves the delivery quality of MPEG video streams while avoiding the waste of network resources, even under heavy load conditions.