Dissociated testicular cell suspensions from 5, 10, 15, 20 and 25 day postnatal rats were cultured either on solid agar as a substratum or in a flask under slow rotation. Spherical or cord-like aggregations were formed from cells under both conditions. Sections from the aggregations showed that they were comprised mainly of Sertoli cells, with peritubular cells located around the aggregations. A few germ cells were present within those aggregations from cultures of rat testicular cells younger than ten days, while no germ cell could be found within aggregations from rats older than fifteen days. Based on these observations, we conclude that testicular cells in post-natal rats still maintain a limited capacity for cell adhesion and recognition and are thus able to form cellular aggregations.