Phosphatidyl serine (PS) was reacted with dimethylaminoanpthalenesulfonyl (DNS) chloride to produce a fluorescent phospholipid derivative (DNS-PS). By comparison of the emission maximum of the fluorochrome in solvents of varying dielectric constants and in aqueous suspension it was concluded that the DNS moiety of sonicated DNS-PS micells was within the glycerol region of the lipid. Addition of CaC! 2 to aqueous suspensions of DNS-PS micells caused a shift in the emission maximum from 524 nm to 490 nm, a two-fold increase in the relative fluorescence and an increase in the polarization of fluorescence and finally aggregation. MgCI 2 produced similar but smaller changes in the fluorescence parameters. In the presence of calcium the thermal transition of the fatty acid tail region could be monitored as an increase in the fluorescence intensity of the DNS group. Without calcium an almost linear decrease in relative fluorescence was observed. These data suggested that calcium caused the fluorochrome to be ~tifted from the glycerol region of the lipid structure into the fatty acid tail region. These observations indicate how the conformation of a biological membrane might be altered by the interaction between acidic pho~pholipids and calcium.