We have studied ganglioside alterations and their enzymatic basis during the course of neural differentiation of mouse embryonic carcinoma cell line P19. This cell line can differentiate into neurons and astrocytes on cell aggregation after treatment with retinoic acid (RA) or into muscle cells on dimethyl sulfoxide (DMSO) treatment. GD3, detected on immunostaining after thin-layer chromatography (TLC) with monoclonal antibody (MAb) R24, was markedly present in aggregates treated with RA.GM3 synthase (alpha2,3-sia1yltransferase, SAT-I) in neurons was found to exhibit the highest activity. GD3 synthase (alpha2,8-sialyltransferase, SAT-II) and GD3 synthase mRNA, as analyzed by Northern blotting, were also markedly present in aggregates and neurons induced by RA.However, on treatment with DMSO, which induces muscle cells, there was no change in the level of GD3 synthase activity, and its transcript was hardly detected during the course of muscle differentiation. GTlb synthase (alpha2,3-sialyltransferase, SAT-IV) was present at similar levels in undifferentiated cells and aggregates treated with RA, but a higher level was observed in neurons. On the other hand, the level of GQlb synthase (alpha2,8-sialyltransferase, SAT-V) in RA-induced aggregates was significantly higher than that in neurons. These results show that RA but not DMS0 induces the expression of GM3, GD3, GTlb and GQlb synthases, and particularly GD3 synthase mRNA, in the ganglioside biosynthetic pathway during the neural differentiation of embryonic carcinoma P19 cells.