Histamine is a physiological mediator which exerts both effector and regulatory functions through its receptors on various cells. The aim of the study was to investigate changes in histamine receptor expression on peripheral blood lymphocytes affected by stimulation with both specific and nonspecific stimuli. Lymphocytes were obtained from both healthy and allergic subjects. Cells were incubated with various allergens (mixed grass pollen, Lolium perenne, Dermatophagoides pteronyssinus 1, bee venom, phospholipase A2) and nonspecific (fMLP, PMA/ionomycin, LPS) stimuli. The percentage of histamine-binding cells was determined with a fluorescence microscope after incubation with histamine-fluorescein. In control subjects histamine binding after stimulation with allergens was not significantly changed. In contrast, in allergic subjects stimulation with specific allergens resulted in significantly increased histamine binding. Nonspecific stimulation caused increased histamine binding to lymphocytes in both allergic subjects and healthy controls. We conclude that specific and nonspecific activation of lymphocytes is associated with increased expression of histamine receptors.