The trend of outsourcing in industries has led to a situation in which various service providers are working at industrial sites. Managing such complex multi-employer worksites is a challenge. This study introduces a collaboration procedure for supplier audits in the context of integrated health, safety, environment, and quality management by large Finnish industrial buyers. A supplier-audit database is analyzed and supplemented by interviews with the buyers. Altogether, 456 audit observations are categorized in seven thematic areas to introduce what kinds of development areas suppliers have. With regard to the collaborative nature of the procedure, the benefits from the buyers' side were identified from the interviews. Cluster collaboration has proven advantageous in financial savings, objective assessments, and supplier development.