Erythema Toxicum, a rash frequently present in the healthy newborn infant is an innate, immune response to the first commensal micro flora. Flushing and urtication are seen in this manifestation suggesting mast cell (MC) activation and MC derived mediator release. It has recently become evident that MCs participate in the protective, innate immune response against microbes also by secreting products toxic to pathogens such as cathelicidin peptide antibiotics. We hypothesized that MCs contribute to the process of inflammation in Erythema Toxicum and that skin MCs of human newborns express the cathelicidin peptide antibiotic LL-37. Skin sections were immunostained for MC tryptase. Double immunofluorescence was performed by staining LL-37 in combination with tryptase. We studied ultra structure of skin MCs with transmission (TEM) and immunoelectron microscopy (IEM). Seven infants with and six infants without the rash, as well as three adults were included. We found numerous tryptase-expressing MCs recruited around the hair follicles in the lesions of Erythema Toxicum. TEM analysis of MCs exhibited signs of degranulation in the lesion. Neither skin MCs from newborns nor adults did express LL-37 as judged by confocal and IEM. MCs participate in the inflammatory responses of Erythema Toxicum by taking an active part in the immune system of the hair follicle. However, their immunological activity is not linked to the expression of the cathelicidin antimicrobial peptide LL-37. A pivotal role of MCs in the innate, inflammatory response at the site of pathogen invasion during the critical time of perinatal colonization is suggested.