The effect of the extract from caudate-putamen nucleus of newborn rats (CPe) on the dopaminergic (DA) neurons has been studied. An MTT colorimetric microassay measuring optical densities for the growth of DA neuron cultures reveals a significant increase in growth for cultures with CPe as compared to those without CPe (p < 0.05). Rhodamine retrograde-prelabelled DA neurons were cultured on a Phastgel containing all the electrophoretically separated protein bands from CPe, and able to fish out from it their own trophic factor, a 12.5-kD protein band. The survived neurons at the 12.5-kD protein band were immunostained positive with anti-dopamine antibody. Co-culturing the 12.5 kD-containing gel strip with explants from substantia nigra at a close distance for a week revealed anti-dopamine immunopositive neurites outgrowing from the explants only towards the 12.5-kD gel strip. These results indicate that a 12.5-kD protein from the CPe is capable of maintaining the survival of DA neurons of the substantia nigra and promoting their neurite outgrowth.