A multicast radio data communication system comprising a master (1) and a plurality of slave stations (2, 3, 4), and using a retransmission protocol. Data is transmitted by the master station and received by the slave stations. One slave station is designated the primary slave station, and all other slaves are designated secondary slave stations. Any slave station may transmit a negative acknowledgement but only the primary slave may transmit a positive acknowledgement. Positive and negative acknowledgements are transmitted substantially simultaneously. Negative acknowledgements corrupt reception of the positive acknowledgement by the master station, thereby ensuring that the data is retransmitted. The primary slave station is selected to have the lowest quality radio link, which may be ensured by a power control command from the master.