The binding of Src to phospholipid membranes requires both hydrophobic insertion of its myristate into the hydrocarbon interior of the membrane and nonspecific electrostatic interaction of its N-terminal cluster of basic residues with acidic phospholipids. We provide a theoretical description of the electrostatic partitioning of Src onto phospholipid membranes. Specifically, we use molecular models to represent a nonmyristoylated peptide corresponding to residues 2-19 of Src [nonmyr-Src(2-19); GSSKSKPKDPSQRRRSLE- NH2] and a phospholipid bilayer, calculate the electrostatic interaction by solving the nonlinear Poisson-Boltzmann equation, and predict the molar partition coefficient using statistical thermodynamics. The theoretical predictions agree with experimental data obtained by measuring the partitioning of nonmyr-Src(2-19) onto phospholipid vesicles: membrane binding increases as the mole percent of acidic lipid in the vesicles is increased, the ionic strength of the solution is decreased, or the net positive charge of the peptide is increased. The theoretical model also correctly predicts the measured partitioning of the myristoylated peptide, myr-Src(2-19); for example, adding 33% acidic lipid to electrically neutral vesicles increases the partitioning of myr-Src(2-19) 100-fold. Phosphorylating either serine 12 (by protein kinase c) or serine 17 (by cAMP-dependent protein kinase) decreases the partitioning of myr-Src(2-19) onto vesicles containing acidic lipid 10-fold. We investigated the effect of phosphorylation on the localization of Src to biological membranes by expressing fusion constructs of Src's N terminus with a soluble carrier protein in COS-1 cells; phosphorylation produces a small shift in the distribution of the Src chimeras from the plasma membrane to the cytosol.