In this paper we present practical experiences gathered from the employment of two popular Java-based mobile-agent platforms, IBM's Aglets and Mitsubishi's Concordia. We present some basic distributed computing models and describe their adaptation to the mobile-agent paradigm. Upon these models we develop a set of frameworks for distributed database access over the World-Wide Web, using IBM's Aglets and Mitsubishi's Concordia platforms. We compare the two platforms both quantitatively and qualitatively. For the quantitative comparison, we propose, employ, and validate an approach to evaluate and analyze mobile-agent framework performance. For the qualitative assessment, we present our observations about the programmability and robustness of, and mobility provided by, the two platforms.