Receptor specific antibodies are excellent probes for a wide range of biological investigations on receptor structure and function. The hybridoma technology (Kohler and Milstein, 1975) has inevitably had a major impact on this field with most of the better known receptors now identified with monoclonal antibodies. This volume of the Receptors and Recognition series provides reviews of recent developments in this field and emphasizes in particular the new opportunities afforded by the judicious application of monoclonal reagents. It is assumed that most readers will be familiar with the now fairly routine methods of cell fusion, hybridoma cloning and selection for producing monoclonal antibodies and so few details of the basic technical procedures are described. Several good reviews on this topic are however available (see Galfre and Milstein, 1981; Goding, 1980; Yelton and Scharf, 1981; McMichael and Fabre, 1982). By no means all vertebrate receptor species are discussed here; omissions include antibodies to low density lipoprotein receptors (Beisiegel et al. , 1981; Kita et al. , 1981), prolactin and growth hormone receptors (Friesen etal. , 1982; Simpson et al. , 1983) and the hepayte asialoglycoprotein receptor (Schwartz et al. , 1981; Harford et al. , 1982). Nevertheless the coverage is comprehensive and critical and the individual chapters provided illustrate vividly the rapid progress being made.