During the postoperative period after radical surgery on oesophagogastric (EG) or colorectal (CR) tumours, we have studied prospectively the influence that factors like protein-calorie malnutrition (PCM), the degree of stress and the characteristics of total parenteral nutrition (TPN) may have on the postoperative levels of certain plasma proteins (PP), especially those of rapid turnover, like acute phase reactant proteins (APRP). 44 patients (23 EG and 21 CR) were assigned randomly to receive one of two regimes of isonitrogenated TPN (0.23 g/Kg weight/day) during the first six postoperative days. The only difference between the two regimes was the relationship Kcal/g N2 (Group I Kcal/g N2 = 100, Group II Kcal/g N2 = 150). The percentage of preoperative PCM was 45%. Apart from albumin (A) and transferrin (T), all the PP recovered their preoperative values on the 6th postoperative day, with very significant increases of haptoglobin (HP) and alpha-1-antitrypsin (AAT) (p less than 0.01). Previous MN, the types of surgery or the ratio Kcal/g N2 hardly modified this response. The cumulative nitrogen balance (CNB) was significantly more positive in patients with previous PCM (p less than 0.05), and in those who underwent EG surgery (p less than 0.05) and in those of GI (p less than 0.02). The rate of postoperative septic complications (PSC) was higher in EG and MN patients (p less than 0.01) and was manifested by significantly lower levels of fibronectin (F) and prealbumin (PA) (p less than 0.01). Preoperative values of A, PA and total proteins (TP) have been shown to have predictive value in PSC.