In this study histologic and histochemical examinations were performed on the bioptic material of 30 cases of head and neck tumors and 34 cases of uterocervical carcinomas after treatment with human leukocyte interferon (HLI). Particular attention was paid to the interaction between stromal and neoplastic cells as a possible host response to the treatment with interferon. In addition to tumor biopsies, regional lymph nodes were also examined microscopically. Stromal cells (macrophages, T and B lymphocytes, and granulocytes) were detected by enzyme-histochemical methods. Histologic and histochemical criteria for classification of tumor biopsies into five groups included the frequency, the cellular composition of peritumoral stroma, the grade of macrophage infiltration into the tumor (expressed quantitatively), and cytologic changes, followed by the regression of tumor. The overall appraisal of patients with cervical carcinoma based on histopathologic and chemical criteria showed the positive effect of HLI therapy in 70% of the cases; observations of head and neck carcinoma treated with HLI showed positive effects in 82% of the cases.