In this paper, graph multiset transformation is introduced and studied as a novel type of parallel graph transformation. The basic idea is that graph transformation rules may be applied to all or at least some members of a multiset of graphs simultaneously providing a computational step with the possibility of massive parallelism in this way. As a consequence, graph problems in the class NP can be solved by a single computation of polynomial length for each input graph. Keywords Graph multiset transformation · DNA computing · Transformation units · NP problems · Massive parallelism