Equilibrium density gradient ultracentrifugation of serum low density lipoprotein (LDL) from twelve healthy human subjects was used to separate six subfractions with mean density ranging from 1.0268 to 1.0597 g/ml. Mean corrected peak flotation rate (Sof) measured by analytic ultracentrifugation, and mean particle diameter determined by negative staining electron microscopy, both declined significantly with increasing density of the subfractions. Major differences in chemical composition of the subfractions were noted, including a significantly lower triglyceride content and higher ratio of cholesteryl ester to triglyceride in the middle fractions compared with those of highest and lowest density. Concentration of fraction 2 correlated positively with HDL (P less than 0.01) and negatively with VLDL (P less than 0.001); concentration of fraction 4 correlated negatively with HDL (P less than 0.05) and positively with VLDL (P less than 0.001) and IDL (P less than 0.01). LDL may thus include subspecies of differing structure and composition which might also have different metabolic and atherogenic roles.