Twelve patients with metastatic colorectal cancer received alternating cycles of low immunomodulating doses of alpha-IFN + 5-Fluorouracil (5-FU) or 5-FU alone. Hematological, biochemical and physical evaluation showed that both treatment cycles were well tolerated. However, transient fever and moderate flu-like symptoms were observed following alpha-IFN administration. Treatment with 5-FU alone produced long-lasting inhibition of CD8+ T lymphocytes, but did not depress NK activity (NKA). Combined treatment with alpha-IFN produced a short-term increase of NKA and antagonized the effect of 5-FU on CD8+ cells on day 5 of the cycle. Parallel studies on in vitro models showed antiproliferative effects of 5-FU on PHA-stimulated MNC and confirmed the preferential inhibition of CD8+ cells. Pretreatment with alpha-IFN did not reverse the effect of 5-FU on CD8+ lymphocytes, but partially protected MNC from the toxic effects of the drug. This was presumably due to the cytostatic effects induced by alpha-IFN on MNC before exposure to the cycle-specific antineoplastic agent. This investigation suggests that alpha-IFN could play a positive role in immuno-chemotherapy of colorectal cancer through multiple mechanisms not entirely related to direct antitumor effects of the agent.