An immunohistochemical reaction was used to study the locations of calretinin-positive cells on frontal sections of the anterior part of the mouse cerebral cortex. A previously undescribed population of cells with a characteristic structure was found at the anterior horns of the lateral ventricles. These cells had small (8-10 microm) round bodies giving rise to one and occasionally two nodose processes bearing rare polymorphous spines (PS) and thickenings of irregular shape. The relatively thick primary processes branched into finer processes, which also formed thickenings and spines of different calibers and structures. Calretinin-positive cells with polymorphous spines (CR+PS) were located in the white subcortical matter, in layer VI, and, significantly more rarely, in layer V of the frontal area of the dorsomedial cortex close to the cingulum. In addition, CR+PS cells were present in the rostrodorsal part of the caudate nucleus-putamen complex, in the anterior olfactory nucleus, in the subependymal layer of the dorsolateral angle of the lateral ventricle and, more rarely, in its dorsal wall. In contrast to the situation in mice, CR+PS cells were not present in the brains of other animals (rats, rabbits, cats). CR+PS cells showed no colocalization of calretinin with GABA or other neuronal or glial markers. It is suggested that these cells represent a previously unknown, probably neuronal, type of cell in the mouse forebrain.