In this paper, the authors propose a protocol for secure mining of association rules in horizontally distributed databases. The current leading protocol is that of the researchers. The authors’ protocol, like theirs, is based on the Fast Distributed Mining (FDM) algorithm of the researcher, which is an unsecured distributed version of the Apriori algorithm. The main ingredients in their protocol are two novel secure multi-party algorithms, one that computes the union of private subsets that each of the interacting players hold, and another that tests the inclusion of an element held by one player in a subset held by another.