1. The conditions that promoted the solubilization of particulate lactose synthetase were effective for solubilizing the thiamine pyrophosphatase of the Golgi apparatus but differed from those effective for β-glucuronidase or acid phosphatase of lysosomes. 2. Lactose synthetase-containing particles did not bind Mg2+ or Cs+ ions, suggesting that they are not related to endoplasmic reticulum membranes. 3. Intact lactose synthetase and thiamine pyrophosphatase particles banded isopycnically at a density of 1·143 in a sucrose gradient. The dissociated ‘A’ sub-unit of lactose synthetase, UDP-galactose hydrolase, p-nitrophenyl phosphate acid phosphatase, alkaline phosphatase and phosphodiesterase I were associated with particles of a broad density range from 1·12 to 1·20. Lysosomal enzymes β-glucuronidase, arylsulphatase and β-glycerophosphate acid phosphatase were associated with particles of density 1·20, 1·175 and 1·15 respectively. 4. Rate-zonal sedimentation studies indicated that lactose synthetase particles have S20,w values exceeding 24000s, corresponding to spherical particles of diameter exceeding 5·4×10−5cm. 5. Electron micrographs of lactose synthetase particles purified over 20-fold revealed small spherical bodies (0·1–0·5μ) resembling lysosomes, the smaller of which were attached to membranes, and larger heterogeneous spherical or oval bodies (0·7–1·8μ) resembling lipofuscin secretory granules. 6. The relationship between lactose synthetase particles and the Golgi origin of secretion granules is discussed.

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