The use of database systems in new applications e.g. computer aided design software engineering e t c leads to new requirements for the transaction management. main characteristics of such environments are long duration of transactions and teamwork. in this paper we introduce a concept for long transactions which especially supports the cooperative work of groups on a common set of data the transaction model has two levels: on the one hand the team has to be protected from the outside world on the other hand mechanisms are required which allow controlled teamwork on common objects in the latter case rigid synchronization mechanisms as used in database systems are not applicable. A main concept for the support of teamwork and for design databases in general is a user-oriented version mechanism this paper presents such a mechanism and its integration into the transaction model.