Abstract—Explants of rat cerebellum obtained 12‐24 h after birth were maintained in culture in Maximow assemblies. About 90 per cent of the cultures myelinated after 10–12 daysin vitro. Cultures maintained for varying periods of time were exposed to [35S]Na2SO4; labelled sulphatide was recovered from the total homogenate. Preparation of a subcellular fraction with density properties corresponding to those of myelin indicated that labelled sulphatide appeared in this fraction. Cultures which were poorly‐myelinated always exhibited a lower rate of inccrpomtion than well‐myelmated cultures from littermate animals, but the distribution of labelled sulphatide into the ‘myelin’ fraction was similar in the two groups. The rate of incorporation of [35S]Na2SO4into total sulphatide increased with the duration of the culture, with a low level of incorporation until the seventh dayin vitro, followed by a sharp increase in rate up to the 21st day. This pattern resembles that observed for rat cere