Serial plasma CEA levels have been studied preoperatively (testing A); one day after surgery (B); 10--15 days after surgery (C); 4 (D), 8 (E), 12 (F), 16--18 (G), and 22--24 (H) months after surgery in a series of 45 patients affected by colorectal carcinoma who started soon after surgery a protocol of adjuvant immuno(chemo)therapy with Levamisole and BCG. Postoperative follow-up was from one to 26 months, with 28 patients followed for at least one year. Fourteen patients had recurrences: two of these had false-negative CEA tests, three had persistent high CEA levels after surgery, nine had increasing levels 9--12 months before clinical recurrence; and nine of these 14 patients showed frankly pathologic preoperative plasma CEA levels. Six patients who did not have a recurrence but (both at clinical and instrumental evaluation) who had two consecutive high plasma CEA levels, were put on prophylactic polichemotherapy. The prognostic importance of CEA levels both pre- and postoperatively, the possibility of "modulating" postoperative adjuvant treatments on the basis of CEA levels, and the problem of unexplained fluctuations of plasma CEA levels with the putative metabolic linkages are discussed.