Spin-labeled derivatives of high density (HDL) and low density (LDL) lipoproteins from human serum were prepared with N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)-maleimide. At the low ratios of spin label to protein employed, no changes in circular dichroism or immunochemical reactions were detected. The lipoprotein derivatives, labeled in their protein moieties, exhibited electron spin resonance spectra containing signals of strongly immobilized and weakly immobilized components. The strongly immobilized component was relatively more prominent in high density than in low density lipoproteins. Delipidation or addition of detergent greatly reduced the relative magnitude of the highly-immobilized signal. Reaction with specific antibodies did not alter the signals of spin-labeled lipoproteins or apoproteins. The signals of the reconstituted complex of spin-labeled apoHDL-phospholipid were indistinguishable from those of a spin-labeled apoHDL in the absence of lipid.