This work examines the interplay of inter-personal and inter-organizational trust, two distinct but related concepts, through a theoretic inter-organizational trust-based security model for a multi-agent system information-sharing community. A calculus that mathematically models trust building at the inter-organizational level is at the heart of this model. In inter-organizational or inter-firm exchange, the role of the boundary spanner, an organizational representative, is important in reducing asymmetries that may exist between the two parties. Trust is a crucial component to the dyadic interaction at the inter-personal or boundary spanner level, and the trust established at this level also affects the overall quality of the relationship at the inter-organizational level. Trust, as an aspect of social control, is thus viewed as a more effective mechanism for security in an open, distributed system, like an information-sharing community. The inter-organizational trust-based security model proposed herein represents a soft security approach. It affords several important benefits over traditional hard security mechanisms used in open systems — robustness, scalability, and adaptability. The inter-organizational trust-based model is an important contribution to the computational security community, as other open systems applications of a distributed or pervasive nature could adapt it and realize its benefits. It is also one of a few attempts to model trust building at either the inter-organizational or inter-personal level.