New organ culture methods permit growth of human renal cortical cells fulfilling all known criteria of juxtaglomerular cells. Two types of granules were observed with the electron microscope. These granules showed an increase in size and number during the growth of these cultures. Their presence in cells was demonstrated by Bowie's, basic fuchsin-crystal violet, and fluorochrome staining methods. Puromycin or actinomycin D caused a reduction in the number of cytoplasmic granules, while 1% oxygen in the gas phase of the culture caused an increase. Specific reaction of these granules with rabbit antihuman renin antibody was demonstrated. Other cellular structures did not react with this antibody. Similar granules were observed in the media of hyperplastic, interlobular arterioles but not in normal arterioles or arterioles of other vascular beds. Bundles of myofilaments could be observed between the cytoplasmic granules in cultured cells. Bioassays of renin activity of the supernatant culture medium showed that it contained renin if granulated cells were present in the culture. All juxtaglomerular cells in culture produced granules, but only those cells which grew from kidneys with pathological changes released renin, suggesting that the stimulus for renin release existed in the intact kidney. This stimulus for renin release seemed to be rapidly lost during culture growth.