Cytokines play an important role in activating the immune system against malignant cells. One of these cytokines, interleukin-4 (IL-4) has entered clinical phase I trials because of its immunoregulatory potency. In the present study we report that recombinant human (rh) IL- 4 has major direct antiproliferative effects on one human lung cancer cell line (CCL 185) in vitro as measured by a human tumor cloning assay (HTCA), tritiated thymidine uptake, and counting cell numbers and marginal activity in a second cell line (HTB 56) in the HTCA. This activity could be abolished by neutralizing antibody against rhIL-4. The biological response of the tumor cells to the cytokine is correlated with expression of receptors for human IL-4 on both the mRNA level and the protein level. The responsive cell line, CCL 185, secretes IL-6 after being incubated with rhIL-4. On the other hand, neutralizing antibodies against IL-6 showed no influence on the growth modulatory efficacy of rhIL-4 in this cell line. Furthermore, CCL 185 does not show detectable production of IL-1, tumor necrosis factor alpha or interferon gamma after incubation with rhIL-4. Thus, the response to rhIL-4 is not mediated through autocrine production of these cytokines triggered by rhIL-4. In a next series of experiments some of the cell lines were xenotransplanted to BALB/c nu/nu mice. Subsequently, the mice were treated for 12 days with two doses of 0.5 mg/m2 rhIL-4 or control vehicle subcutaneously per day. Treatment with rhIL-4 yielded a significant inhibition of tumor growth versus control in two of the non-small cell lung cancer cell lines being responsive in vitro (CCL 185, HTB 56). Histology of the tumors in both groups showed no marked infiltration of the tumors with murine hematopoietic and lymphocytic cells consistent with the species specificity of IL-4. In contrast, no tumor growth inhibition was found in the small cell lung cancer cell lines (HTB 119, HTB 120) being nonresponsive in vitro. We conclude that rhIL-4 has direct antiproliferative effects on the growth of some human non-small cell lung cancer cell lines in vitro and in vivo, which together with its regulatory effects on various effector cell populations makes this cytokine an interesting candidate for further investigation in experimental cancer treatment.