In the basophil activation test (BAT), the expression of the degranulation marker lysosomal-associated membrane glycoprotein-3, also termed as CD63, or upregulation of CD203c is determined. Degranulation is the fusion of specific intracellular vesicles filled with preformed me- diators, which are the so-called granules, with the plasma membrane and the transition of CD63 from inside out. The result is a sudden and pronounced rise, i.e., log shift, of the fluorescence intensity signal in the detection of surface CD63 molecule. Concomitantly, the upregula- tion of CD203c has been observed; this can be detected as a significant increase in the mean fluorescence intensity signal of the CD203c detec- tion antibody. The most common identification strategies use surface IgE, eotaxin CC chemokine receptor 3, the interleukin-3 receptor alpha chain CD123, the prostaglandin D2 receptor CRTH-2, or the basophil- specific ectonuclease CD203c.The usefulness of BAT in immediate drug hypersensitivity reactions is highly variable and dependent on the drug and its capacity to spontaneously conjugate to serum proteins. Stimula- tion with pure solutions of the parent drug or metabolites thereof ver- sus drug-protein conjugates may influence the sensitivity and specific- ity of the BAT. Other influencing factors are as follows: the selection of stimulants or of identification and activation markers, the protocol for stimulation, strategies for gating, and the definition of the cut-off. The BAT is helpful to detect immediate drug hypersensitivity reactions to beta-lactam antibiotics, neuromuscular blocking agents, radiocontrast media, platinum-containing chemotherapeutics, analgesics, and bio- logicals or quinolone. In general, although there is a good correlation among the skin test, drug provocation test, and BAT; BAT has proven to be useful to complement in vivo tests.