In the fully contracted metaphase chromosome, the length of DNA is compacted at least 10,000 times, and many details of chromosome organization cannot be distinguished. Study of the more elongated prophase chromosomes permits the observation of finer details; for example, the number of bands in a human haploid chromosome set increases from about 300 in metaphase (1) to as many as 2000 in prophase (2). However, the number of naturally occurring prophases in normal mitotic chromosome preparations is usually very small, and artificial means are required to increase the number of elongated prophase chromosomes to an acceptable level for high-resolution analysis. One method used for high-resolution banding (Chapter 1) is the synchronization of cells in interphase, followed by release of the block and harvesting of the cells an appropriate time later when the cells have progressed through to prophase. An alternative approach, which will be described here, is to add to the cultured cells some substance that will either be incorporated in the chromosomal DNA or, more usually, bind to the chromosomal DNA, and thereby inhibit condensation of the chromosomes. Such substances may either inhibit the condensation of whole chromosomes or of specific parts, normally heterochromatin.