Representative DNA histogram of GPIIbIIIa-positive cells grown in liquid culture. Equine low-density marrow cells were cultured in the presence of Pre-Tp foal serum, labeled with an anti-GPIIbIIIa antibody, stained with propidium iodide, and analyzed by flow cytometry. The position of the 2 N peak was determined by setting markers for peak propidium iodide staining of PBMCs from a normal horse. Three thousand GPIIbIIIa-positive cells were analyzed. Ploidy distribution is similar to that obtained in MK cultures exposed to preinfection serum and normal horse serum.