Twenty-three pyrogenic reactions occurred in 16 patients undergoing hemodialysis at a private dialysis center in the south central United States between November 23 and December 2, 1978. No deaths were attributed to reactions; however, 10 patients were hospitalized for observation after experiencing a reaction. Cultures of all blood specimens obtained from the patients gave negative results. Chills (75 percent), nausea and/or vomiting (30 percent), and fever (90 percent) were the most common signs and symptoms, with mean times of onset after starting dialysis of 1.1, 1.6, and 3.6 hours, respectively. An epidemiologic and laboratory investigation documented that reactions occurred only in patients who had anticoagulation with a dilute solution of heparin. Analyses of heparinized saline solution used during the outbreak revealed a bacterial count of 7.4 X 10(5)/ml and a bacterial endotoxin level of 1,300 ng/ml. Acinetobacter calcoaceticus var. Iwoffi was isolated from the solution. Diluted heparin solution was prepared at the dialysis center by adding commercially supplied sodium heparin to 0.9 percent sodium chloride infusion fluid. Bacteria and endotoxin were not detected in vials of stock heparin and bags of unopened 0.9 percent sodium chloride infusion fluid. We concluded that contamination of the solution occurred at the dialysis center. After changes in the preparation and use of heparin were instituted on December 4, 1978, no pyrogenic reactions occurred in more than 400 subsequent dialyses.