Abstract: Using Laser Microprobe Mass Analysis (LAMMA), we studied the chemical composition of lead-induced intranuclear inclusions in rat kidney tissue prepared by three different wet chemical fixation procedures for transmission electron microscopy. Fixation with glutaraldehyde-Na2S gave the same results as fixation with glutaraldehyde only: a high lead concentration could be detected. Therefore, for lead strongly bound to proteins, precipitation procedures are not essential. Post-fixation with osmium tetroxide drastically changed the composition of the inclusions: the lead concentration decreased substantially, while sodium, calcium, and barium were introduced. The osmium tetroxide fixative was found to be the source of the contamination. It also contained aluminum, and we suggest that other proteins (e.g., in neurofibrillary tangles) might be able to take up Al out of solution and that care must be exercised in interpreting the microanalytical results of osmium-fixed material. For the microanalysis of the lead inclusions, fixation with glutaraldehyde only provides a good compromise between preservation of the ultrastructure and maintenance of the element distribution.