We present an abstraction of the probabilistic semantics of Multiset Rewriting to formally express systems of reactions with uncertain kinetic rates. This allows biological systems modelling when the exact rates are not known, but are supposed to lie in some intervals. (abstract) models we perform probabilistic model checking obtaining lower and upper bounds for the probabilities of reaching states satisfying given properties. These bounds are under- and overapproximations, respectively, of the probabilities one would obtain by verifying the models with exact kinetic rates belonging to the intervals. Key words: probabilistic model checking, systems biology, uncertain rates, abstract interpretation, interval Markov chains.