Methylation of membrane phospholipids is an important stage in the process of histamine release. This methylation reaction can be modulated by S-adenosyl-methionine (SAM) as well as by S-adenosyl-homocysteine (SAH). The influence of SAM and of SAH upon the histamine release of leucocytes from asthmatic and normal subjects were compared. In concentration of 10(-4) M, SAM enhanced spontaneous and the specifically induced histamine release. In concentration of 10(-5) M SAM enhanced the histamine release only in normals. This phenomenon could indicate a deficit in methyl-transferase activity in the asthamatics. In concentration of 10(-4) M, SAH reduced spontaneous and specifically induced histamine release, having an effect comparable to that of 10(-4) M Theophylline.