With the advent of high-speed access network technologies such as ADSL, increasing numbers of Internet users are participating in various interactive multimedia applications. Among these, the most popular are the massively interactive on-line games, or MMPOGs. In MMPOGs, a large amount of event data is associated with various control objects. This event data has different characteristics from that which is generally used on the Internet. Namely, events occur very frequently with short inter-arrival times and their size is quite small, because they only contain control information. Most commercial MMPOGs use TCP or UDP as the transport protocol for the event data. However, since TCP is such a heavy protocol, due to its complex congestion control algorithm and byte-oriented window scheme, it is difficult to support many concurrent users. On the other hand, UDP is a relatively lightweight protocol, but there are no functions available which permit reliable transmission and session management. In this paper, we propose a new transport protocol, Game Transport Protocol (GTP), which is designed for the transmission of the event data used by MMPOGs. GTP supports several functions designed to meet the various requirements of MMPOGs. Firstly, GTP uses a packet-based window scheme not a byte-based window scheme as in the case of TCP. This scheme is quite simple and suitable for the small size of the event data. Also, GTP performs session management and retransmission using GTP control blocks, and supports an adaptive retransmission scheme that controls the maximum number of retransmissions according to the real-time priority, in order to meet the time constraints of the event data. Although GTP is a specialized transport protocol, optimized for MMPOGs, it could also be utilized as a transport protocol for other interactive multimedia applications.