The experiments presented here confirm the hypothesis according to which, in our experimental system of differential cell adhesion (where we studied the kinetics of the earliest period of adhesion of a suspension of chick embryo neuroblasts to layers of astroblasts or fibroblasts), the mechanism of adhesion appears to consist of two steps, the first of which is a short-term reversible phase corresponding to a binding equilibrium. In fact, adhesion of neuroblasts to each of the two cell layers occurs according to kinetic constants and attains levels which are characteristic for each of the two adhesion systems. In both systems, neuroblasts that have not adhered at equilibrium are able to adhere if inoculated over a fresh cell layer of the same type, as they do during the first inoculation; conversely, neuroblasts that have adhered to a cell layer can be made to de-adhere by substituting cell-free fresh medium to the inoculation medium containing non-adhering neuroblasts. This shows that, as predicted for a reversible equilibrium system, removal of adhering neuroblasts from the system at equilibrium provokes adhesion, and removal of non-adhered neuroblasts provokes de-adhesion. Furthermore the level of adhesion at equilibrium is, in all cases, the same. The reversibility of adhesion, which is almost quantitative during the onset of the equilibrium, gradually decreases with time, indicating the presence of a process of irreversible attachment between cells after the first reversible step. The developmental implications of the complete sequential mechanisms are briefly discussed.