We have begun a biochemical-genetic analysis of the synthesis of sphingolipid long-chain bases in Saccharomyces cerevisiae and found evidence for the occurrence of serine palmitoyltransferase (SPT) and 3-ketosphinganine reductase, enzymes that catalyze the initial steps of the pathway in other organisms. SPT activity was demonstrated in vitro with crude membrane preparations from S. cerevisiae as judged by the formation of radiolabeled 3-ketosphinganine from the condensation of palmitoyl-coenzyme A (CoA) with radiolabeled serine. Shorter (C12 and C14) and longer (C18) acyl-CoAs sustain significant SPT activity, a result consistent with the finding of both C18 and C20 long-chain bases in the organism. Three products of the long-chain-base synthetic pathway, 3-ketosphinganine, erythrosphinganine, and phytosphingosine, neither directly inhibited the reaction in vitro nor affected the specific activity of the enzyme when these bases were included in the culture medium of wild-type cells. Thus, no evidence for either feedback inhibition or repression of enzyme synthesis could be found with these putative effectors. Mutant strains of S. cerevisiae that require a sphingolipid long-chain base for growth fall into two genetic complementation groups, LCB1 and LCB2. Membrane preparations from both lcb1 and lcb2 mutant strains exhibited negligible SPT activity when tested in vitro. Step 2 of the long-chain-base synthetic pathway was demonstrated by the stereospecific NADPH-dependent reduction of 3-ketosphinganine to erythrosphinganine. Membranes isolated from wild-type cells and from an lcb1 mutant exhibited substantial 3-ketosphinganine reductase activity. We conclude that the Lcb- phenotype of these mutants results from a missing or defective SPT, an activity controlled by both the LCB1 and LCB2 genes. These results and earlier work from this laboratory establish that SPT plays an essential role in sphingolipid synthesis in S. cerevisiae.