The interaction of the covalently reacting probes dansyl chloride, fluorodinitrobenzene and trinitrobenzene sulphonic acid with isolated central nerve myelin sheath preparations has been studied. The three probes interact preferentially with accessible amino groups on lipid and protein in the membrane. With isolated myelin some 13% of the total phosphatidyl ethanolamine is labelled with dansyl chloride while the figure is 66% with fluorodinitro benzene and 47% with trinitrobenzene sulphonic acid. Lower levels of phosphatidyl serine are labelled. Phosphatidyl ethanolamine seems to be more accessible to probes in the myelin sheath than is phosphatidyl serine perhaps because the ethanolamine-containing lipid class is localised partially at the external apposition surfaces of the membrane which are most accessible to the probes. The serine phospholipids may not react so well because they are preferentially distributed at the cytoplasmic surface of the system. Analysis of protein labelling patterns after reaction of intact myelin with dansyl chloride indicates that the high molecular weight proteins and the proteolipid protein is accessible to the probe while the basic protein is not, even though this latter component is readily labelled with dansyl choride in purified form. It is suggested that the inability of the basic protein to react with myelin is perhaps due to the fact that it is occluded from interaction with the probe at the cytoplasmic apposition surfaces of the lamellae.