Primary solid tumors were mechanically and/or enzymatically disassociated, and the resulting suspensions of single cells and small clumps of cells were seeded onto three different substrates, i.e., tissue culture plastic, rat smooth muscle cells (SMCs), and SMC-derived extracellular matrix. Tests of the relative effectiveness of these substrates in supporting the survival and/or growth of ten different neoplasms demonstrated that only two explants remained viable for longer than 2 weeks when seeded onto tissue culture plastic while nine of the ten survived on biological substrates for 1 month or longer. Thus, tissue culture plastic was a poor substrate for primary pediatric neoplasms. In general, more than 80% of the most common solid neoplasms in childhood (brain tumor, neuroblastoma, renal tumor, rhabdomyosarcoma, osteogenic sarcoma, and Ewing's sarcoma) routinely survived or grew in long-term cultures when cultured onto SMCs or their matrix. Both substrates were effective in promoting survival and/or growth; however, cells of neuroblastomas and certain brain tumors showed a preference for a living smooth muscle substrate. Tumor cells maintained their characteristic cellular and subcellular morphology when compared with the histology of the in vivo neoplastic lesions. Light and electron microscopy of selected neoplasms cultured on SMCs for various time periods demonstrated areas of distinct cellular invasion and/or partial destruction of the SMC multilayers which correlated with the invasive potentials of the neoplasms in patients. Invasion and destruction of the SMCs were also noticed with quiescent tumor cell cultures, indicating that growth was not a necessary property of invasion. Several neoplasms were also capable of the degradation of connective tissue proteins as indicated by the hydrolysis of radiolabeled SMC matrices, but simple correlations between the extent of matrix degradation and invasive ability could not be drawn. The culture system described consistently provided for the survival and/or growth of the most common pediatric tumors for long time periods. Thus, basic biological properties of primary tumors, e.g., growth, invasive potentials, and differentiation capabilities, could be investigated routinely.