Most of the current security architectures for grid systems use conventional public key infrastructure (PKI) to authenticate grid members and to secure resource allocation to these members. Certificateless public-key cryptography (CL-PKC) has some attractive properties which seem to align well with the demands of grid computing. In this paper, we present a certificateless protocol for authentication and key agreement (CL-AK) which fits well with the Grid Security Infrastructure (GSI) and provides a more lightweight key management approach for grid users. We show that the newly proposed protocol is of great efficiency and practical. Moreover, we prove that it provides perfect forward secrecy plus all the other security attributes of authentication and key agreement protocols such as known-key secrecy and no key control.