Summary.-C3H and C57 BL/6 mice carrying methylcholanthrene-induced fibrosarcomas were injected i.p. daily with histamine, metiamide (anti-histamine type-2 receptor), histamine + metiamide, mepyramine (anti-histamine type-i receptor), serotonin and methysergide (anti-serotonin). Inhibition of tumour growth and lengthened survival were observed with histamine and histamine+ metiamide. The best results (both on tumour growth and survival) were obtained with serotonin. Survival was increased by metiamide and decreased by mepyramine and methysergide. In histamine-treated and in serotonin-treated mice, histological studies of the tumours showed large and numerous foci of haemorrhagic necrosis.Stimulation of histamine type-I or serotonin receptors and inhibition of histamine type-2 receptors play a beneficial role in the host's defence against tumours.