This study has confirmed that patients who have advanced colorectal carcinoma have impaired responsiveness to delayed-hypersensitivity skin testing, and also have elevated levels of serum IgM. Serial observations of delayed-hypersensitivity skin tests, total lymphocyte counts, T-lymphocyte counts, B-lymphocyte counts, and serum immunoglobulin levels failed to reveal any consistent pattern of responses in patients treatment with either chemotherapy alone or chemoimmunotherapy. In 33 patients chosen at random to receive either 5-FU alone or 5-FU in combination with intramuscularly administeredC. parvum, there was no evidence of objective response or influence on survival. Intramuscularly administeredC. parvum, in the dose and schedule used, produced no evident immunologic or therapeutic effect.