The reports contained in this volume of abstracts comprise three cate­ gories of papers presented at the Third International Congress of Histo­ chemistry and Cytochemistry. The format of the meeting called for a series of Plenary Sessions on each of the five mornings of the Congress as follows: I. Recent Developments in Cytochemical Localization by Electron Microscopy. 2. Centrifugal and Electrophoretic Techniques in Histochemistry. 3. New Directions in Quantitative Cytochemistry by Physical Techniques. 4. Subcellular Structural-Functional Correlates. 5. Current Developments of Histochemistry in Pathology. All the reports in these sessions were by invitation. Four of the five afternoons were devoted to contributed papers from the platform, some of which represent invited reports. A third category rep­ resents papers submitted "by title". In the present volume the three categories are not distinguished one from the other. All papers are listed alphabetically by the last name of the first author-the only division being papers received too late for in­ clusion in the main body of abstracts and published at the back of the volume. The position of specific papers in the overall program of the Congress and the names of session cl1airmen are listed in the separate program volume of the meeting. All editorial work relative to the Congress, and specifically this volume, were undertaken by the staff of the Office of the Secretariat. In this re­ spect I am grateful to the fine organizational and editorial work of Mrs.