In malignancies, an interruption of growth (dormancy) is sometimes observed. In the immunogenic mammary carcinoma MC2, dormancy followed by regression after a period of growth was observed in 18% of s.c. implants in normal mice. Dormant implants removed for histologic examination were invariably found to be completely surrounded by a highly fibrous stroma. Fibrosis was enhanced in immunized mice, and reduced in immuno-deficient mice. Surgical disruption of the fibrous capsule around dormant tumors early (19 +/- 3 days) in the immune response led more frequently to resumed growth, while later (32 +/- 3 days) disruption of the capsule led more frequently to complete regressions. This showed that fibrous capsules that could destroy tumors could also shield them against well-developed systemic immune mechanisms.