Two cases of transfusion-related Serratia marcescens bacteremia prompted extensive epidemiologic investigations in three independent hospitals. Test tubes and plasma from donors whose blood was drawn into bags from a single production batch were cultured. Analysis of the ribotype of S. marcescens isolates was performed. For comparison, a strain from the production plant and eight other, unrelated bacteremia isolates were examined. In addition, a retrospective national survey was carried out. S. marcescens was cultured from 11 (0.73%) of 1515 blood units, and an additional (third) bacteremic patient was identified. The clinical isolates from three patients, the three units of blood transfused, and the plant-derived strain shared a unique ribotype. The incident is interpreted as a sporadic, bacterial contamination of blood bags with the S. marcescens epidemic strain, occurring during the manufacturing or packaging. A similar incident has not previously been reported. Attention is drawn to the possibility of significant contamination during the complex production of multiple-bag blood collection systems. Guidelines for improved registration and handling of transfusion complications in wards are suggested. Manufacturers should be encouraged to provide blood packs with sterile exteriors, in appropriate, single, outer packages.