A new variation of the antimonate precipitation technique, employing tannic acid in the primary aldehyde-antimonate fixative, is described for use in the subcellular localization of calcium in various tissues. Chelation studies and electron microscopic, X-ray microanalytical studies of antimonate precipitates in etiolated oat tissues indicate that calcium is the major cation localized using the present experimental protocol. Preservation of ultrastructural morphology in these tissues is greatly improved over that observed in tissues fixed with conventional antimonate-aldehyde or antimonate-osmium fixatives. The regularity and reproducibility of tissue precipitate patterns suggests that 1) penetration of the tissue by the fixative, and subsequent precipitation of calcium, is rapid and uniform and 2) ion displacement during sample preparation is negligible. Calcium appears to be immobilized efficiently in situ, with greater than 90% 45Ca retention in radiolabeled tissues prepared for electron microscopy. Quantitative aspects of calcium precipitation by antimonate in 45Ca-labeled CaCl2 solutions were examined over a wide range of calcium concentrations. Precipitation was essentially linear over the expected range of biological concentrations of calcium. Furthermore, the 3:1 antimonate to calcium ratio estimated for test tube precipitates was also established for Sb/Ca in tissue precipitates analyzed using energy dispersive x-ray microanalytical (EDX) techniques. These observations suggest that the present technique is potentially useful in the semiquantitative estimation of tissue calcium levels.