Cell lines derived from human sarcomas have been previously shown to produce a protein growth factor which, similar to platelet-derived growth factor (PDGF), can induce competence for mitosis in fibroblasts. Whether this factor production is an important feature of sarcomas in vivo or simply an artefact caused by long-term culture conditions is unclear, however. We demonstrated growth factor activity in conditioned medium from six of 11 primary sarcoma cultures, utilizing flow cytometric analysis of DNA to monitor the presence of sarcoma cells in cultures. Cells isolated from control tissues failed to show a similar mitotic effect.