Human osteosarcoma (HOS) clonal cells transformed in vitro by N‐methyl‐N'‐nitro‐N‐nitroso‐guanidine (MNNG) were characterized, and compared to non‐producer HOS cells transformed by Kirsten murine sarcoma virus (Ki‐MSV). The MNNG‐ and virus‐transformed cells grew in the aggregate form above an agar base, grew in soft agar, and had a high fibrinolytic activity. When inoculated into nude mice, all the chemically or virally altered cells produced tumors or tumor nodules. When transplanted into ATS‐treated hamsters, the cells transformed by MNNG (0.01 μg/ml) and Ki‐MSV produced tumors but MNNG (0.1 μg/ml) transformed cells did not produce tumors. The control HOS cells did not grow in the aggregate form but formed colonies in soft agar, and had low fibrinolytic activity and no capacity to form tumors in nude mice and ATS‐treated hamsters. However, one of the control clonal lines had a high level of fibrinolytic activity. Cellular aggregation properties of human transformed cells did appear to correlate with tumorigenicity in nude mice.