The methods of immunolabeling make visible the presence of
specific antigens, proteins, genetic sequences, or functions
of a cell. In this paper we present examples of imaging
immunolabels in a scanning transmission x-ray microscope using
the novel method of dark-field contrast. Colloidal gold, or
silver-enhanced colloidal gold, is used as a label, which strongly
scatters x-rays. This leads to a high-contrast dark-field image
of the label and reduced radiation dose to the specimen. The
x-ray images are compared with electron micrographs of the same
labeled, unsectioned, whole cell. It is verified that the
dark-field x-ray signal is primarily due to the label and the
bright-field x-ray signal, showing absorption due to carbon,
is largely unaffected by the label. The label can be well
visualized even when it is embedded in or laying behind dense
material, such as the cell nucleus. The resolution of the images
is measured to be 60 nm, without the need for computer processing.
This figure includes the x-ray microscope resolution and the
accuracy of the label positioning. The technique should be
particularly useful for the study of relatively thick (up to
10 μm), wet, or frozen hydrated specimens.