During a 7 year period, 16 episodes of VM-26 (teniposide) hypersensitivity occurred in our Department of Pediatrics. Eight of these (50%) were observed in neuroblastoma patients, of whom a total of 22 children had been treated with VM-26. The predominant signs were facial edema, flushing, urticaria, bronchospasm, tachycardia, and hypotension. All children with hypersensitivity recovered, but four of them were critically ill. No risk factors were found. In order to elucidate the mechanism of the hypersensitivity episode further, and to identify a possible allergen, histamine release from basophil leukocytes was performed by use of a glass microfiber method. Blood samples from nine cases reacting to VM-26, eight controls (children exposed to VM-26 without any hypersensitivity reactions), and 12 healthy children without previous exposure were challenged with VM-26 alone and with its vehicle, cremaphor. In all samples, it was found that VM-26 degranulated basophils, whereas no histamine release was seen after challenge with cremaphor. The reaction was dose-dependent, and not IgE-mediated, since IgE depletion of the cells did not abolish histamine release after VM-26 challenge.