A series of 200 human tumors were cultivated in vitro in an attempt to establish cell lines. Lines were established, with explant and trypsinization techniques, from 13 tumors including carcinomas, sarcomas, melanomas, and brain tumors. All these lines, In culture for over 1 year, exhibited marked refractility, multilayering, and criss-crossing and were morphologically distinct from normal contact-inhibited human fibroblast or epithelial lines. They also formed colonies on monolayers of normal cells and grew with a high efficiency in soft agar. Preliminary results indicated abnormal chromosomal patterns in all lines tested, and 8 of 9 cell lines formed tumors in antithymo~yte serum-treated mice. The rate of establishment (approximately 6%) of lines from random neoplastic material demonstrated that cells with properties of transformed cells could be recovered from tumor tissue, but it also emphasized the need for improved methodology in this area.