The widespread diffusion of metasystems and grid environments makes it necessary to employ programming models able to well exploit a high, variable number of distributed heterogeneous resources. Many software frameworks designed for Grid computing do not address this problem. They only allow the use of existing programming libraries based on explicit message-passing communication models, often not suitable to manage the variability of a Grid. In this paper we present the customization of a component-based middleware for metacomputing, HiMM (Hierarchical Metacomputer Middleware), in order to support distributed programming based on the Active Object model provided by ProActive. This way a meta-system can be efficiently and transparently programmed by unifying the asynchronous remote method invocation model and the reflection provided by meta-objects.