Sphingolipid long-chain base phosphates (LCBPs) regulate cell proliferation, movement and differentiation in higher eukaryotes. To study the function of LCBPs in Saccharomyces cerevisiae, we inactivated LCBP breakdown pathways. Elimination of both the Dpll lyase and the Lcb3 phosphatase pathways by gene deletion was lethal, indicating that these enzymes regulate LCBP levels to prevent accumulation. Lethality was prevented by eliminating the major LCB kinase. Lcb4p, which synthesizes LCBPs, but not by eliminating the minor LCB kinase, Lcb5p. These data imply that death results from an accumulation of LCBPs made by the Lcb4p kinase. By regulating Lcb4 kinase activity, we found that cell death correlates with LCBP accumulation and that C18 dihydrosphingosine-l-P (DHS-P) and C20 DHS-P are most likely the killing molecules. LCB levels were found to be most elevated in a strain lacking Lcb4 kinase, Dpll lyase and Lcb3 phosphatase activity. Analysis of mutant strains suggests that the C18 and C20 species of LCBPs are preferentially degraded by the Lcb3 phosphate phosphatase, while the Dpll lyase prefers C16 DHS-P as a substrate. These and other data indicate the existence of an unknown mechanism(s) for regulating LCB levels. Our results demonstrate that LCBPs may be used in some circumstances to regulate yeast cell growth.