Intercellular communication in higher organisms depends on the central nervous system and hormones. Simple organisms such as the cellular slime molds communicate intercellularly only by using hormone-like signals. The most intensively studied species of the cellular slime molds is Dictyostelium discoideum. Aggregating cells of this species secrete cyclic AMP as chemoattractant, and very low concentrations of this intercellular communication signal induce molecular, behavioral and developmental changes in neighboring cells. The transduction of such a signal in the responding cell has several characteristics in common with hormone action. Binding of cyclic AMP to the cell-surface receptors of the responding cell is specific, rapid, saturable and reversible. The activated receptor regulates internal cGMP and CAMP levels and, as after hormone activation, calcium fluxes, methylation, refractory periods and down regulation are observed. Moreover, the synthesis of key enzymes might be a response to chemotactic signals. Dictyostelium, as well as being simple, is also a suitable organism to grow in large quantities and, experimentally, it is a favorable subject because the amebae are activated by chemoattractants in their single-cell phase. A large variety of mutants that may be blocked somewhere between the beginning and the end of the transduction process is available. When the cells are still free, uptake of food takes place by phagocytizing bacteria. Also, the vegetative cells respond to simple chemoattractants, such as folic acid (Pan et al., 1972), that are secreted by the bacteria. Specificity for chemotactic signals is not so important in this stage because the amebae feed on almost any bacterial species (Raper, 1937). After the food supply is exhausted and there is no gradient of chemoattractant secreted by bacteria, the cells themselves start to secrete more specific chemotactic