We have investigated possible interactions of divalent-cation-requiring processes and cytochalasin-sensitive processes in the early events of lymphocyte activation. In lectin-stimulated responses, Ca and Mg were synergistic in support of protein and DNA synthesis, as expressed in the increased requirement for one ion under conditions of depletion of the other. Experiments evaluating the temporal relationships between cytochalasin sensitivity and requirements for Ca and Mg demonstrated that both cations were required before or during the early cytochalasin B-sensitive events. Furthermore, we observed that Ca and Mg and cytochalasin B-sensitive processes were required for lectin-dependent commitment to DNA synthesis. Additional studies comparing the relative potencies of cytochalasin E, D, and B suggested that the probable target for cytochalasins in the inhibition of commitment was a motility-related process. These data demonstrate an early period of activation which can be characterized by its requirement for divalent cations and its sensitivity to cytochalasins. © 1981.