Surgery for primary breast cancer and regional nodes often involves the use of suction drains post-operatively. It is believed that the fluid accumulating following axillary surgery is lymph from divided lymphatics or alternatively serum, although the composition of this fluid has never been studied in detail. Drain fluid following surgery for primary breast cancer was analysed for a number of haematological and biochemical parameters. Comparison of these values with plasma concentration produced a ratio that was compared with established lymph to plasma ratios. Haematological analysis indicates minimal contamination with blood. Comparison of drain fluid to plasma ratios with known lymph to plasma values for biochemical parameters showed that this fluid is compositionally different from lymph, but is similar to inflammatory exudate. Altering surgical technique to close dead space may avert the need for wound drainage, lead to reduced hospital stay and reduce the incidence of 'seroma' formation.