Cell transplantation therapy typically involves transplanting primary cells or immortalized cells into patients. The promising but still inconsistent data stemming from those clinical trials using primary cells in Parkinson's disease are believed to be due to an insufficient number, function and uniformity of the transplanted cells. In an effort to overcome these problems an improved method for isolating, growing and differentiating precursor cells into dopaminergic neurons has been developed. The process described provides for an expansion of the cell number of primary cells by up to 1000 fold. This technique could assist in solving the problem of obtaining sufficient cells for a reliable, effective cell transplantation therapy. The process consists essentially in the isolation and in vitro numerical expansion of an early mesencephalic precursor population, the use of serum, cAMP, dopamine and ascorbic acid during differentiation and the development of an aggregation technique during cell differentiation that allows convenient grafting of dopaminergic neurons.