Purpose. To examine the levels of Muller cell contraction-stimulating activity in human vitreous, correlate these levels with clinical presentation, and identify the causative growth factors. Methods. Human vitreous was collected from patients undergoing pars plana vitrectomy (n = 84). Muller cells were isolated from porcine retina and maintained in tissue culture. Tractional forces generated by cells incubated on three-dimensional collagen gels were measured as changes in gel thickness. Contraction- stimulating activity in vitreous (VA) was calculated from the dose-response profiles of gel contraction to vitreous protein. The contributions of individual growth factors to vitreous activity (n = 10) were assessed by inhibition with specific neutralizing antibodies. Results. The mean VA of patients with retinal detachment (3.65) and proliferative vitreoretinopathy stages A, B, and C (2.06) were elevated above that of patients without retinal pathology (vitreous activity = 0.23) or retinal defects alone (0.57). Mean activities in patients with epimacular proliferation (1.22) and vitreous hemorrhage (1.40) were also significantly elevated. The percentage of this activity attributable to insulin-like growth factor 1 (IGF-1) varied from 9.2% to 84.5% with a mean of 61.3%. Similarly, the percent contribution of platelet-derived growth factor (PDGF) ranged from 6.8% to 49.0% with a mean of 26.5%. Conclusions. The vitreous of patients with retinal detachment, proliferative retinal disease, and vitreous hemorrhage contain varying amounts of growth factors that stimulate tractional force generation by Muller cells. The majority of the activity can be attributed to IGF-1 and a smaller proportion to PDGF.