Abstract: The present disclosure provides compositions and methods for stimulating, enriching, expanding, and/or activating engineered cells that express a recombinant receptor, e.g., a chimeric antigen receptor. In some embodiments, the provided methods include ex vivo or in vitro stimulation, enrichment, expansion, and/or activation of cells by incubation with a particle, e.g., a bead particle, attached to a binding molecule, such as a polypeptide antigen or anti-idiotype antibody, that recognizes or binds to the recombinant receptor. Also provided herein are methods for transfecting or transducing cells that have not been previously incubated with an activating or stimulating agent, such as have not been incubated with anti-CD3/anti-CD28 antibodies and/or one or more recombinant cytokines, by transducing or transfecting the cells in the presence of the particles with attached binding molecules. In some embodiments, the provided compositions can be used in methods to prepare cells, e.g.