Dopaminergic cell therapy is a potential viable treatment for Parkinson's disease. However, lack of a well-characterized cell preparation of known phenotypic composition containing a high percentage of dopaminergic neurons, has prevented a definitive, controlled, pilot clinical trial from being conducted. We report the successful in vitro expansion of rat E12 mesencephalic progenitors to produce 5-fold the normal number of dopaminergic neurons. The expanded neurons (MAP2+) were detached, resuspended, and formed into small aggregates of 10-200 neurons containing 25-50% of dopaminergic neurons (TH+) that will likely be optimal for use in successful cell therapy. After storage in DPBS, in 0 mM Ca(2+) for up to 24 h at room temperature, aggregated cells were still 90% viable. These results demonstrate that it might be feasible to use a similar protocol to expand human dopaminergic progenitors in vitro. If successful, the requisite large numbers of dopaminergic neurons required to conduct a pilot clinical trial for Parkinson's disease will be produced in vitro. Indications are that the cells can be maintained at optimal viability for the duration of the neural transplantation procedure, under real operating conditions.