A simple method is described by which hybridomas can be selected and cloned in a single step immediately after fusion. This is done by plating the cells in semi-solid medium containing methylcellulose and the components of the HAT selection system. A number of variables have been examined in order to optimise the technique. The system is particularly suitable for isolating large numbers of hybridomas secreting different monoclonal antibodies. Evidence is presented to show that the colonies which grow in the system are in all probability clones. Thus, the need for routine recloning of the hybridomas is eliminated. In this way, the technique cuts down on the amount of tissue culture work associated with the production of monoclonal antibodies. Using this technique, it is easier to plate out large numbers of cells and to recover many independent hybridoma clones, than is the case when using cloning by limiting dilution.