The northern leopard frog, Rana pipiens, may be afflicted with a herpesvirus-transmitted renal carcinoma which has the interesting property that its metastatic behavior is temperature-related. PNKT-4B is a cell line derived from a pronephric carcinoma arising in a tadpole. We sought to ascertain if invasion of normal tissue by PNKT-4B cells in three-dimensional confrontation culture in vitro is similarly temperature-dependent. Normal fragments of tadpole and frog organs are invaded by PNKT-4B cells at 28 degrees C but not at 20 degrees C or 21 degrees C. PNKT-4B cells failed to invade tadpole tissues at 7 degrees C. A temperature critical for invasion was sought. Temperatures of 21 degrees C and cooler are invasion-restrictive and 23 degrees C and warmer are invasion-permissive under the conditions of this study. Identification of a critical permissive temperature allows for the characterization of biochemical events which may be activated at the same temperature. The biochemical changes, which are selectively activated and subsequently repressed as tumor cells are cycled through invasion-permissive and invasion-restrictive temperatures, become compelling candidates as reactions involved in, or causal for, malignant invasion.