Tryptases secreted by tissue mast cells and basophils can enter the bloodstream. In human subjects tryptases are encoded by several genes and alleles, including ⍺, β, γ, and δ. Common variations include complete absence of ⍺ genes. Until recently, ⍺ tryptase was considered to be the major tryptase secreted at baseline and in mastocytosis. However, lack of ⍺ tryptase genes has little effect on circulating tryptase levels, which are now thought mainly to consist of inactive pro-β tryptase secreted constitutively rather than stored in granules with mature tryptases. Pro-β tryptase levels thus might reflect total body mast cell content. In contrast, mature β tryptase can increase transiently in severe systemic anaphylaxis and confirm the diagnosis. However, it might fail to increase in food anaphylaxis or might increase nonspecifically in samples acquired after death. Thus pro- and mature β tryptase measurements are useful but associated with false-negative and false-positive results, which need to be considered in drawing clinical conclusions in cases of suspected anaphylaxis.