The techniques of nuclear transplantation and microinjection have been used to develop systems for investigating the interrelationship of cytoplasm and nucleus in determining gene expression. The systems which will be analyzed are: gene expression in chick erythrocyte nuclei reactivated by introduction into mouse fibroblast cytoplasts; gene expression in mouse fibroblast nuclei introduced into cytoplasts from "differentiated" cell lines such as hepatoma, neuroblastoma and adrenal cortex; nuclear gene expression and changes in cytoarchitecture, mitochondrial functioning and growth control in cytoplasmic-nuclear hybrids constructed from cytoplasts and karyoplasts prepared from two cells of two different species (mouse and human); viral gene expression and expression of the transformed pehnotype in hybrids constructed from cytoplasts and karyoplasts prepared from SV40 and RSV transformed and revertant cell lines. With each system, we seek to answer the following questions: How do the hybrids resemble or differ from the parent cell types? Does the cytoplasm alter nuclear gene expression? Does the nucleus reorganize the cytoplasm? What factor(s) or characteristics of the cytoplasm induce observed change in nuclear gene expression? Are these changes in expression reflected in structural changes in the chromatin? The methodology employed to approach these questions involves a combination of the techniques of molecular and cellular biology: analysis of specific gene expression using labeled cDNA; analysis of cytoarchitecture using immunofluorescence with antisera prepared against myosin, actin and tubulin; analysis of expression of differentiated functions using a variety of histochemical and biochemical assays; etc. Knowledge gained concerning the mechanisms regulating gene expression in eukaryotic cells should prove of fundamental importance in understanding the phenomena of differentiation and transformation.