The developmental pattern of gangliosides in human fetal brain should reflect the activities of the respective glycosyltransferases. LA2-synthase activity, along with that of GM3-, GD3-, GM2-, and GM1-synthases, was determined in human fetal brain at 10-22 weeks of gestation. LA2-synthase is the pivotal enzyme in lacto series ganglioside formation. LA2-synthase activity decreased during the study period, mirroring a similar temporal decline in levels of the lacto series gangliosides, particularly 3'-isoLM1. The developmental profiles of the ganglio series glycosyltransferase activities demonstrate distinct changes that correspond to the ganglioside pattern between fetal weeks 10 and 22. In particular, the marked increase in GM2-synthase activity at 20 and 22 weeks of gestation and the decline in GD3-synthase activity after 15 weeks could explain the prominent expression of the a series gangliosides in this period of rapid neuronal outgrowth. However, a similar decline (two- to 2.5-fold) in GM3-synthase activity suggests a more likely conclusion, namely, that the two sialyltransferase activities are derived mainly from astroglial cells, which show a marked proliferation during the 10-15th fetal weeks. The data do not negate the hypothesis that GM3- and GD3-synthase are the critical enzymes in the regulation of ganglioside biosynthesis but do indicate a need to reevaluate the significance of GM2-synthase in expression of the a series gangliosides.