In this paper, we present a framework for Medium Access Control (MAC) protocol development and performance evaluation. The framework, developed for the Rice University Wireless Open-Access Research Platform (WARP), allows us to interface a large class of medium access protocols with custom physical layer (PHY) implementations, thereby providing a flexible and high-performance research tool. MAC protocols for our framework are written in C and targeted to embedded PowerPC cores within the Xilinx Virtex II-Pro class of FPGAs. A key innovation is a flexible interface between the PHY and the MAC capable of exposing user-defined parameters to either layer, thus enabling cross-layer research.