In recent years a number of different techniques have been developed to fuse cells of various origin in order to obtain hybrids with qualities different from each of the original cells. In many cases, one wants to fuse large numbers of cells (e.g. to generate hybridomas), and fusion occurs more or less randomly (e.g. Kohler and Milstein, 1975). With other methods, individual cells have been fused under controlled conditions (e.g., McGrath and Solter, 1984). So far a prerequisite for fusing two preselected cells has been that they must be isolated away