Endothelial cell growth factor(s) from several previously untested human tumor cell lines (i.e., SK-HEP-1, MG63, A375, TE671-C1, RD) were detected using a low cell inoculum growth assay. The final cell density in the 2-cm2 wells was determined by a highly sensitive DNA content measurement performed directly in the tissue culture plates. The sensitivity of the assay to human tumor cell growth factors depended critically on the low cell inocula, 2,000 to 5,000 cells/well. Most of the bovine endothelial cells used were cloned from primary cultures; all the cell lines obtained from various fetal and nonfetal sources responded to the growth factor(s) (up to a 16x stimulation) as well as to endothelial cell growth supplement. Dose response curves showing the cell specific response of bovine endothelial cells were obtained. The growth stimulatory activity and the in vivo chick embryo chorioallantoic membrane assay responses correlated sufficiently to imply that the assay is detecting tumor angiogenesis factor or some closely related activity. This in vitro assay should prove useful in the identification and purification of tumor-derived factors and in the elucidation of the role of these factors in the events comprising angiogenesis.