Experimental studies were performed to investigate further the effects of immunotherapy with Propionibacterium avidum KP-40 on thymocyte proliferation, maturation and emigration in BALB/c-mice. Thymus weight and thymocyte counts, especially cells presenting the immature or cytotoxic/suppressor phenotype were significantly increased. Due to enhanced emigration, peripheral blood lymphocyte and monocyte counts as well as expression of activation markers were significantly upregulated. The antimetastatic effect of Propionibacterium avidum KP-40 was demonstrated in BALB/c-mice, where RAW 117-H10 lymphosarcoma liver colonization was significantly reduced after immunostimulation. Clinical investigations proved that surgical treatment of colorectal carcinoma induced an evident decrease of peripheral blood lymphocytes as compared with preoperative counts. However, single preoperative Propionibacterium avidum KP-40 administration induced a considerable increase of peripheral white blood cell counts, especially lymphocytes. Clinical effects of preoperative immunostimulation by Propionibacterium granulosum KP-45 were investigated in a prospective randomized trial in colorectal carcinoma patients. Positive effects on survival time, local tumor recurrence and distant metastasis could be demonstrated in stages I and II, whereas no advantage of immunotherapy was found in advanced stages III and IV. A recent prospective randomized clinical trial was initiated on the quality of life of colorectal carcinoma patients. Three months after surgical treatment negative effects could not be determined after immunotherapy. Quality of life even proved to be better in patients with abdominoperineal resection as compared to non Propionibacterium avidum KP-40 treated control patients.