In order to improve the perioperative resistance to the spread of cancer during operation the effect of preoperative stimulation of the immunesystem by Propionibacterium granulosum KP-45 was investigated in patients with colorectal carcinoma. In a prospective randomized trial 101 patients were allocated to either treatment (n = 51) or control (n = 50). In the treatment 10 mg of Propioni bacteria were administered intravenously between the seventh and third day prior to surgical treatment. At the time of operation 21 tumours were classified as stage I (treatment n = 12, control n = 9), 22 as stage II (treatment n = 10, control n = 2). Postoperatively wound infections requiring treatment were more prevalent in the control group (n = 4) than in the treated group (n = 0). All patients were subsequently followed up for 76 months. For stage I carcinoma the survival rates, excluding perioperative deaths, were 91% in the treated and 63% in the control group respectively. One case of tumour metastasis was seen in the control group. For stage II carcinoma the survival rate was 90% for the treated group with distant spread in 1 case and 45% in the control group where the rate of recurrence was 55%. For stages III and IV there was no statistically significant difference in survival between the treated and the control groups.