Large numbers of animal cells can be enucleated by centrifugation in the presence of cytochalasin. The nuclei (minicells) obtained by this technique are surrounded by a thin rim of cytoplasm and a cell membrane. The cytoplasms (anucleate cells) contain cytoplasmic organelles and are capable of protein synthesis. By themselves, these cell fragments are not viable under usual culture conditions for longer than 2 days after enucleation. They can, however, be fused by UV-inactivated Sendai virus to form reconstituted cells. Nuclei from rat myoblasts have been combined with cytoplasms from mouse fibroblasts. The resulting reconstituted cells are viable and multiply rapidly in vitro. Enucleation or disruption of "micronucleated" cells, cells in which the genome has been fragmented into many small micronuclei by prolonged exposure to antimitotic agents results in microcells. These cell fragments are subdiploid and may contain as little as a single chromosome. Fusion of microcells from normal cells with intact mutant cells offers a method of transferring complementing chromosomes into genetically defective cells. Specific areas in which reconstitution techniques should prove useful include studies of gene expression, cell differentiation, and the dependence of mitochondria on nuclear genes. Microcell hybrids could be useful in chromosome mapping and gene complementation analysis.