Today, neither transactional provisions, in general, nor concurrency control, in particular, of DBMS-based processing are tailored to the specific needs of large and collaboratively used XML documents. Existing DBMSs more or less offer strictly serialized operations on them. To gain great progress in this area, we have implemented the XTC testbed as an (O)RDBMS-connected native XML database management system to empirically explore finegranular concurrency control on XML documents which is, at the same time, adjusted to the specific needs of their APIs. In this paper, we give a comprehensive description of locking protocols for direct and navigational access to individual nodes of XML documents. Then, we focus on the implementation concepts for our lock manager which realizes this functionality in XTC, before we characterize its performance behavior on a rich spectrum of locking protocols using selective measurements in real applications. 1 Motivation XML applications dramatically grow...