The influence of cell density and cell contacts on the proliferation of neuroblasts in culture and its stimulation by meningeal extract were investigated. Dissociated brain cells from 6-day-old chick embryos were cultured under 3 different culture conditions to obtain dense or sparse brain cell cultures, as well as cultures of isolated neuronal cell. The proliferation fo neuroblasts, shown by morphological observations, cell counts, determinations of DNA content and measurements of [3H]thymidine incorporation, was found to be the highest in cultures where cell density and cellular contacts were greatest. The addition of meningeal extract stimulated the multiplication of neuroblasts only in cultures where the cells were in closer with each other. The results suggested, therefore, that cell density and cell-cell interactions are of importance and favored neuroblast proliferation.