  Semantic equivalences are used in process algebra to capture the notion of
similar behaviour, and this paper proposes a semi-quantitative equivalence for
a stochastic process algebra developed for biological modelling. We consider
abstracting away from fast reactions as suggested by the Quasi-Steady-State
Assumption. We define a fast-slow bisimilarity based on this idea. We also show
congruence under an appropriate condition for the cooperation operator of
Bio-PEPA. The condition requires that there is no synchronisation over fast
actions, and this distinguishes fast-slow bisimilarity from weak bisimilarity.
We also show congruence for an operator which extends the reactions available
for a species. We characterise models for which it is only necessary to
consider the matching of slow transitions and we illustrate the equivalence on
two models of competitive inhibition.
