Authentication and roaming are two critical issues for the integration of heterogeneous networks, such as the integration of WLAN access networks and mobile cellular networks. Due to the strong points of mobile cellular networks, it is favorable to integrate the authentication mechanism of WLAN access networks into that of mobile cellular networks. For GSM/GPRS networks, the subscriber identity module (SIM) card is used for user identification, authentication and message encryption. Therefore, it is feasible to authenticate the subscribers in WLAN via exchanging the authentication information between mobile cellular networks and subscribers' SIM cards. In this work, the issue of the subscriber authentication for WLAN access networks is investigated. By integrating the authentication mechanism of WLAN into that of GSM/GPRS networks, a GSM/GPRS SIM-based authentication mechanism for WLAN as well as the related protocols is proposed. Furthermore, the implementation issues are well investigated, and an experimental system is implemented to verify the feasibility of this authentication mechanism.