Abstract Relative changes in the amount of galactocerebroside (GC) during development were measured in mouse brain cell cultures at different stages of development. For this purpose we used an 125I-labelled protein A indirect assay modified in the respect that the total amount of cellular proteins was evaluated before counting the radioactivity. The amount of GC greatly increased between the 10th and the 14th day of culture, then a steady state was reached between the 14th and the 20th day of culture. This change correlates well with the dynamics of the number of oligodendrocytes we observed earlier. These data suggests that the increase of the GC amount in culture during development corresponds to the increase in the number of GC-positive oligodendrocytes rather than to the increase in the number of GC molecules per cell.