Optical Burst Switching (OBS) has been proposed as an efficient technology to cope with the ever increasing bursty traffic on the Internet. The emergence of a broad range of network-driven applications (e.g. multimedia, online gaming)introduces the need for a network environment able to provide service with different Quality of Service (QoS) guarantees. Burst loss probability, as one of the main QoS metrics on bufferless OBS networks, plays a considerable role on providing a network service with a particular level of QoS. In this paper, a novel MAC based protocol is proposed to support multiple QoS levels on OBS networks without wavelength conversion. It has been evaluated through extensive simulations and its simplistic form makes it very promising. Results indicate that the protocol can clearly provide a relative differentiation of services and guarantee null (or negligible) burst loss probability for a wide network (or offered) load range and, finally, low access delay for the highest priority traffic.