Cultured cells from 15 soft tissue and bone tumors and surgical specimens from 14 of these were studied by electron microscopy. The cells, cultured for one to five weeks, maintained ultrastructural features similar to those found in the fresh tumor tissue. The ultrastructural characteristics of the cultured cells were distinct enough for diagnostic purposes. Short-term cultures from all tumors were subjected to cytogenetic analysis. Clonal chromosome aberrations were found in 11 of the tumors and normal karyotypes in the remaining four. Ultrastructural analysis of cultured cells can be used for cell type identification and tumor classification.