Abstract: A multicast radio data communication system comprises a master and a plurality of slave stations, and uses a retransmission protocol. Data (10) is transmitted by the master station simultaneously to all slave stations. Each slave station acknowledges in turn in a respective time period (13 to 16) which may be assigned by the master station or pre-set. If the master receives a positive acknowledgement from each slave station, the master proceeds to transmit new data. If the master receives a negative acknowledgement from any slave station, or fails to successfully receive any acknowledgement from any slave station, the master station retransmits the previously transmitted data.