Tannin accumulations in tissue culture cells of slash pine occurred in the smooth endoplasmic reticulum, in vesicles, and within cell vacuoles. Dense osmiophilic granules were observed also throughout the cytoplasm of many cells. Vacuolar deposits appeared as homogeneous spherules or as a froth-like material. Generalized degeneration of the cytoplasm and the organelles was observed in many of the cells synthesizing tannins. Accumulation of the tannin within the cell vacuoles did not necessarily result in cell degradation. Synthesis and deposition of tannin in slash pine cells closely resembles synthesis and storage of aromatic products in mammalian tissues.