The 5-methyl-BDY-3-dodecanoic acid (B12FA) labelling of BHK cell lipids was analyzed by thin layer and reverse phase column chromatography. Incorporation to phospholipids was selective: over 90% of B12FA label was enriched in phosphatidylcholine. The major molecular species of PC was that containing palmitate as the unlabelled fatty acid. Small amounts of label was also found in other phosphoglycerides, but not in sphingomyelin. Triglycerides and diglycerides constituted the main B12FA-labelled neutral lipid classes; however, no label was found in cholesterol esters. B12FA was degraded to shorter homologues, which had significantly slower lipid incorporation rates. B12FA-labelled cells displayed in a microscope initially green reticular type fluorescence, but later red spherical structures, representing neutral lipid droplets, could also be seen. It is concluded that B12FA does not incorporate indiscriminately to all lipid classes of BHK cells, but is enriched to PC, diglycerides and triglycerides, which could be utilized in studies on lipid transport as well as metabolism.