Dictyostelium provides a biochemically and genetically accessible system for studies of transmembrane signaling. Current techniques to control cellular sensitivity and monitor chemotaxis, cyclic adenosine monophosphate (cAMP) signaling, and adenylate cyclase activation are outlined. Receptor-binding assays are reviewed, and a protocol for receptor purification is offered. The assays are brought together in a flow sheet for characterization of transmembrane signaling mutants. Transmembrane signaling mutants can be simply isolated by screening for morphological aberrations during development. The differentiated phenotype is reversible so that, once isolated, the mutants can be grown and propagated as the wild type. A cell surface cAMP receptor is identified and purified, and a specific antiserum is raised. Evidence suggests that this receptor may regulate chemotaxis, activation of adenylate cyclase, and gene expression. An extensive, cAMP-mediated phosphorylation of the receptor may cause adaptation of the physiological responses. The recent demonstration of in vitro guanylnucleotide regulation of the adenylate cyclase suggests that control mechanisms, similar to those found in vertebrates, may operate in this primitive eukaryotic organism. © 1987, Academic Press Inc.