This article reports the results of adjuvant immunotherapy studies carried out in an experimental tumor model system. A transplantable murine BALB/c colonic adenocarcinoma (C-26) was used to evaluate the therapeutic potential of treatment with recombinant interleukin 2 (rIL-2) at high and low dosage in combination with or without lymphokine activated killer cells (LAK) or tumor-specific immune lymphocytes, either as an adjuvant treatment in mice bearing spontaneous post-surgical metastases or in mice bearing artificial metastases. Moreover the in vivo LAK activation and circulation pattern have been examined during or after treatment. These results show that cytotoxic lymphocytes from tumor-immunized donors are more active than LAKs when given in combination with rIL-2 as an adjuvant treatment, while all treatment modalities exerted a strong but generally transient inhibition on the formation of artificial lung metastases by C-26 cells. Alteration of host lymphocyte responsiveness to rIL-2 was shown to occur in early-tumor-resected and large-tumor-bearing animals in both in vitro and in vivo experiments. Moreover, no evidence of homing or preferential localization of the adoptively transferred LAK cells to the site of tumor growth was observed.