We have studied the ultrastructure of histamine-containing cells in the bronchial lavage of normal rhesus monkeys. These cells were recognized by the similarities of their specific cytoplasmic granules to those in tissue mast cells and basophilic leukocytes of other species. According to cell shape, two forms of bronchial lavage histamine-containing cells (BL-HCC) were observed. One was round and small, and the other pleomorphic and large. However, the subcellular structures, especially the specific cytoplasmic granules, were identical for both forms of HCC. With available evidence, we now consider both types of BL-HCC to be one cell species. The difference in cell shape may be an inherent variation within this cell population, or a reflection of the maturity of individual cells. Most BL-HCC had long slender pseudopods, and the majority of them were mononucleated. Some HCC appeared binucleated; multinucleated cells were not encountered. The ultrastructure of mast cells, in tissue fragments biopsied from three sites, was in many ways similar to that of BL-HCC, but there were clear morphological dissimilarities between these two cell groups. It is unlikely that tissue mast cells are the direct precursors of BL-HCC. Attempts to compare BL-HCC with blood basophilic leukocytes were unsuccessful due to the failure to detect the latter in monkey peripheral blood with electron microscopy in the present study. The possible origin of BL-HCC is discussed.