Invasion, destruction and replacement of normal tissues by cancer cells is the first critical step in the metastatic cascade and is the result of complex interactions between tumor and host factors. In an attempt to understand the complex mechanism of local invasion, we have developed a simple, reliable and generally applicable in vitro system using the confrontation of precultured heart fragments (PHF) with a constant known number of potentially invading (10(5)) cells. This grading system is based essentially on quantitative data standardized both for the portion of the explant invaded by the neoplastic cells and the type of invasion demonstrated. Using this system we could measure the ability of an aggressor cell to (a) adhere or attach to PHF (Grade 1), (b) invade the outer fibroblastic layers (Grade II) and/or the cardiac muscle cells (Grade III) and (c) destroy and completely replace the PHF (Grade IV). It was at once apparent from these experiments that, irrespective of their invasive capacity and/or their neoplastic state, both malignant (SKBR-2 III, BT-20, MCF-7, ZR-75-30 LoVo and YAC-1) and non-malignant (HBL-100) cells attach to the PHF. Only malignant cells, however, showed substantial local invasion of both the outer fibroblastic layers and/or the cardiac muscle cells. Most important for the actual problems of invasion in vivo, is the fact that malignant cells from different cell lines demonstrate a wide range in their invasion capacity: three different patterns of invasion were thus established; a highly invasive, a slowly invasive and a poorly invasive pattern. We also show that invasion and proliferation, as defined for the purposes of this study, are two different and independent properties of a given cell line. SKBR-2 III and YAC-1 are here shown to possess the most aggressive potential; they both invade the PHF very early and completely. The rapid proliferation of these aggressive cells and the destruction of the host tissue lead to the rapid disappearance of the myoblasts and their complete replacement by the invading cells. Non-malignant epithelial cells attached to but could not invade even the outer fibroblastic layers of the PHF.