In practice, it is very important to determine the size of a proposed software system yet to be built based on its requirements, i.e., early in the development life cycle. Given a size estimate, it is usually possible to estimate the effort that will be needed to build this system. The most widely used approach to size estimation is Function Point Analysis (FPA). It is not clear, however, how function points can be reasonably counted for object-oriented requirements specifications. We found that this cannot be done fully automatically, since several constructs of such a representation can be interpreted in various ways in the spirit of FPA, depending on the context. For applying FPA to object-oriented requirements specifications, we defined rules that specify a semi-automatic transformation from an object-oriented requirements model to an FPA model.