Most live cells are naturally poorly visible and so various secondary techniques, such as staining (including fluorescent tagging), are used to visualize them. However, a major limitation of most staining techniques is that they commonly compromise cell vitality and/or functionality because the stain either covalently attaches to functional molecules or has toxic interactions. In contrast, FSL (function-spacer-lipid) constructs are designed to be dispersible in biological media, they can insert into cell membranes, and a range of synthetic molecules can be attached, such as fluorophores, without affecting the cells functionality or vitality (1–3). Two constructs for fluorescent tagging of live cells were investigated. In the first, FSL incorporated FITC as its functional moiety (FSLFITC), and the second approach used biotin (FSL-biotin), which was then secondarily reacted with fluorophore labeled avidin. Both approaches were used to successfully label a variety of living cells including murine embryos, spermatozoa, epithelial and endometrial cells. FSL-FITC was additionally used to stain the digestive tract and circulation of zebra fish embryos. Acknowledgement: Supported by KODE Biotech Ltd (kodebiotech.com).