Among all the different architectures being researched in the field of multi-agent systems, the mobile agent has shown to be one of the most challenging and most critical systems. With more applications being developed, there is a need to ensure large and complicated mobile agent systems are functioning correctly, with minimum or no errors. Moreover, the model-based testing technique has gained attention with the popularization of models in software design and development. Since the paradigm on nets within nets is well suited to express the dynamics of open mobile agents, it is retained as an abstract model from which abstract test cases are generated. Those test cases are then concretized and addressed to the system under test. The responses of the system under test are, finally, compared to the expected results derived from the abstract test model. As a case study, we modeled a packet world example on which different colored packets are scattered. Agents that live in this virtual world have to collect those packets and bring them to their right destination. Keywords: mobile agents, nets within nets, reference net, renew, testing. For further information, please visit this web site.