This chapter discusses the isolation of DNA from pig spleen, where almost all cells contain DNA (deoxyribonucleic acid). Some tissues contain high deoxyribonuclease activity so that the DNA is broken down into smaller fragments. A convenient source for the isolation of DNA should therefore contain a high quantity of the material and have low deoxyribonuclease activity. Lymphoid tissue is very good in these respects and thymus is the best source, with spleen as a good alternative. Chop fifty grams of pig spleen into small fragments and homogenize with two hundred millilitre of buffered saline for one minute. Remove any sediment by centrifugation and stir the solution continuously with a glass rod while adding an equal volume of distilled water. Weigh the dry DNA and dissolve by continuously stirring in buffered saline diluted one in ten with distilled water and store frozen until required.