When the level of granularity of services approaches that of business activities, humans become part of a service-oriented system not just as users but as providers of services. A model of such a system has to take into account the characteristics of human actors as service providers. Conversely, in the world of agent-based systems, software components have been attributed with human properties such as reactivity, autonomy and proactivity. We believe that modelling techniques developed for software agents are a valid starting point for specifying human agents in service-oriented systems (HASOS). In particular, we extend UML use case and class diagrams by concepts of role based access control (RBAC) and use graph transformation (GT) rules to model changes to data as well as the dynamic (re-)assignment of roles played by human actors. From these models we can derive specifications of the services required systematically in terms of pre- and post-conditions as well as communication scenarios modelling their interactions. We use the formal framework provided by GT to formalize consistency relations between the different parts of these models. The technique will be demonstrated with the use of a pharmacy scenario.