To analyse the composition of the serous fluid formed after axillary dissection.Descriptive study.University hospital and teaching hospital, The Netherlands.16 patients whose axillas were dissected as part of a modified radical mastectomy for stage I or II breast cancer.Chemical and cellular composition of axillary drainage fluid on the first, fifth, and tenth postoperative days compared with the same constituents in blood and with reported data on the composition of peripheral lymph.On the first postoperative day the drainage fluid contained blood contents and a high concentration of creatine phosphokinase (CPK). After day one it changed to a peripheral lymph-like fluid but containing different cells, more protein, and no fibrinogen, making coagulation impossible. The reduction in the fluid production must be caused by other wound healing processes, such as formation of scars and connective tissue.