We investigated the peritumoral and intratumoral immune infiltrate in 6 basal cell carcinomas (BCCs) treated with recombinant alpha 2b-interferon. Each BCC was injected intralesionally three times a week for 3 weeks with 1.5 x 10(6) IU of interferon per injection (total dose 13.5 x 10(6) IU). The immunohistological study was done before the start of interferon therapy and 15 days afterwards, using a series of monoclonal antibodies and an immunocytochemical technique. Before therapy the infiltrate consisted mainly of CD3+ (T) cells, with prevalence of CD4+ (helper/inducer) T cells. The percentage of T cells expressing interleukin-2 receptor (CD25+ cells) was higher in the tumor nests than in the peritumoral infiltrate (20% and 11% respectively). CD1+ (Langerhans) cells and CD14b+ cells (monocytes/macrophages) were present in the peritumoral infiltrate in all cases (9% +/- 5% and 14% +/- 7% respectively). Very few CD56+ (natural killer), CD15+ (granulocytes) and CD20+ (B) cells were observed in the peritumoral infiltrate and none at all in tumor nests. After 15 days of interferon therapy, we observed an increase in peritumoral and intratumoral CD4+ cells. There was a decrease in the number of CD25+ cells and of CD1+ cells in the peritumoral infiltrate. The number of intratumoral CD25+ increased. No variations were seen in CD14b, CD15, CD20, and CD56 positive cells. Eight weeks after completion of therapy, two BCCs were cleared and the remaining four showed clinical and histological improvement. These results may indicate a direct effect of interferon against BCC; in addition the immunohistological findings suggest that intralesional interferon enhances T cell mediated immune response, especially in tumor nests.(ABSTRACT TRUNCATED AT 250 WORDS)