Component-based software development (CBSD) is one of the main programming paradigms of the last decades. The main idea of CBSD is to modularize a system as a configuration of multiple interacting components. Components interact with each other via dedicated component interfaces hiding a component's implementation and making components interchangeable. In this paper, we present an early investigation of unit testing practices of open source component-based software systems with the goal to find out how component-based software systems are actually tested and how to improve unit testing practices as part of future research. Our preliminary results show that unit tests typically directly test the component implementation and not dedicated component APIs. The method coverage of component APIs varied between 17% and 34% in the analyzed projects.