Allergic diseases such as asthma have been on the rise in recent decades. Environmental or occupational exposure to estrogenic synthetic chemicals is suspected to be a contributing factor, and previous experimental studies indicated that estradiol and some xenoestrogens increase allergic signaling responses, such as degranulation, in immune cells. In the current study, data showed that the estrogen mimetic 4-tert-octylphenol (4tOP) enhances immunoglobulin (Ig) E-mediated degranulation of mammalian mast cell line RBL-2H3 (RBL). At the noncytotoxic concentrations 10-20 μM, 4tOP significantly increased degranulation in antigen (Ag)-activated RBLs but exerted no marked effect on spontaneous levels. Our data suggest that the industrial chemical 4tOP has the potential to enhance allergic disease in individuals who are exposed.