Abstract: The disclosure is directed to a method for seed train expansion of adherent cells comprising culturing cells with a serum-supplemented growth medium in a N-2 vessel; removing the cells from the serum-supplemented medium; inoculating the cells from step into a serum-free growth medium in a N-1 vessel; culturing the cells in the N-1 vessel under suspension conditions; and inoculating a growth medium in a bioreactor with the suspension-cultured cells. In some aspects, the adherent cells are not suspension-adapted. In some aspects, the adherent cells are suspension-adapted. In some aspects, the adherent cells produced by the seed train expansion method are used to produce viral vectors. In some aspects, the viral vectors are AAV vectors.