Capitalizing on our experience in chromosome in situ hybridization, we have embarked on a new procedure designed to detect the expression of particular genes in tissue sections in situ. Using DNA or RNA probes, we have begun to analyze immunoglobulin, T cell receptor, and oncogene expression in cytocentrifuge preps of characterized cell lines, as well as in normal and abnormal lymph node specimens. The research and clinical utility of this technique would, we feel, be profound if it could be as easily and systematically applied as are current histochemical and immune cytochemical techniques.