To establish a useful laboratory protocol to investigate possible cases of fatal anaphylaxis, we measured mast-cell-derived tryptase levels and allergen-specific immunoglobulin E (IgE) antibody levels in sera obtained prior to or within 24 h after death from 19 anaphylaxis victims. Elevated serum tryptase levels (range = 12 ng/mL to 150 micrograms/mL) were found in nine of nine Hymenoptera sting fatalities, six of eight food-induced fatalities, and two of two reactions to diagnostic therapeutic agents. Tryptase levels were normal (less than 10 ng/mL) in 57 sequential sera obtained postmortem from six control patients. Tryptase could not be measured in pleural or pericardial fluids for technical reasons. Serum IgE antibodies were elevated in five of the nine Hymenoptera sting fatalities and in eight of the eight fatal food reactions; assays were unavailable for the two diagnostic/therapeutic agents. If elevated, the victim's serum IgE antibodies to food could be used to identify allergens in uneaten portions of foods consumed shortly before the anaphylactic event. IgE antibodies were moderately stable during storage in a variety of anticoagulants at room temperature for up to 11 weeks. Elevated mast-cell-derived tryptase levels in postmortem sera reflect antemortem mast cell activation and may be used as a marker for fatal anaphylaxis. If assays are available for IgE antibodies to relevant allergens, such assays provide evidence for antemortem sensitization; these assays may be modified to identify allergens in foods consumed by victims of food-induced anaphylaxis.