High IgE responder rats were sensitized intraperitoneally with alum-adsorbed ovalbumin and Bordetella pertussis organisms. At day 0, 7, 14, 21 and 28 following sensitization the peritoneal cells were harvested and further processed for light and immunoelectron microscopical investigations using a specific anti-rat IgE immunogold sandwich method. The results obtained show that the number of peritoneal mast cells increase significantly during the course of sensitization. There is a time-dependent increase in the amount of immunogold particles on mast cell surfaces together with a shift of particle distribution towards the surface folds. Sensitized mast cells exhibit altered releasability as is indicated by slightly degranulated cells at day 21 and day 28 postsensitization. Additionally, about 25% of small lymphocytes which had entered the peritoneal cavity between day 7 and day 14 are heavily stained with the immunogold complex between day 14 and day 28 postsensitization. This is not so for macrophages (less than or equal to 0.2% positive cells) nor for eosinophils which do not show any staining activity at all despite they are present in high numbers.