['In mast cell (MC) neoplasms, clinical problems requiring therapy include local aggressive and sometimes devastating growth of MCs and mediator-related symptoms. A key mediator of MCs responsible for clinical symptoms is histamine. Therefore, use of histamine receptor (HR) antagonists is an established approach to block histamine effects in these patients.', 'We screened for additional beneficial effects of HR antagonists and asked whether any of these agents would also exert growth-inhibitory effects on primary neoplastic MCs, the human MC line HMC-1, and on two canine MC lines, C2 and NI-1.', 'We found that the HR1 antagonists terfenadine and loratadine suppress spontaneous growth of HMC-1, C2, and NI-1 cells, as well as growth of primary neoplastic MCs in all donors tested (human patients, n = 5; canine patients, n = 8). The effects of both drugs were found to be dose-dependent (IC(50): terfenadine, 1-20 μM; loratadine, 10-50 μM). Both agents also produced apoptosis in neoplastic MCs and augmented apoptosis-inducing effects of two KIT-targeting drugs, PKC412 and dasatinib. The other HR1 antagonists (fexofenadine, diphenhydramine) and HR2 antagonists (famotidine, cimetidine, ranitidine) tested did not exert substantial growth-inhibitory effects on neoplastic MCs. None of the histamine receptor blockers were found to modulate cell-cycle progression in neoplastic MCs.'] Do h1-receptor antagonists terfenadine and loratadine inhibit spontaneous growth of neoplastic mast cells?