C18 Phytoceramide-d3 (t18:0/18:0-d3) is intended for use as an internal standard for the quantification of C18 phytoceramide (t18:0/18:0) by GC- or LC-MS. C18 Phytoceramide (t18:0/18:0) (Cer(t18:0/18:0)) is a bioactive sphingolipid found in S. cerevisiae, wheat grains, and the stratum corneum layer of mammalian epidermis. Cer(t18:0/18:0) is composed of a phytosphingosine backbone amine-linked to a C18 fatty acid chain. Cer(t18:0/18:0) has a role in regulation of apoptosis, cell differentiation, proliferation of smooth muscle cells, and inhibition of the mitochondrial respiratory chain. It also inhibits expression of the allergic cytokines IL-4, TNF-α, and transcription factors c-Jun and NF-κB in histone-stimulated murine skin tissue. Formulations containing cer(t18:0/18:0) have been used used in cosmetics as a skin protectants as they reduce water loss to prevent epidermal dehydration and irritation.