We have begun to characterize the development of the excitation – response coupling sequence in the human promyelocytic leukemia cell line HL60. Using the recently developed fluorescent calcium probe quin‐2, it was found that DMSO induced myeloid differentiation of the HL60 cells is accompanied by the development of a calcium response to the addition of the chemotactic factors fMet‐Leu‐Phe and leukotriene B4. The characteristics (time course, concentration dependence, stereospecificity, and metabolic dependence) of the calcium response are extremely similar to those previously described in human neutrophils. These results imply that functional receptors for leukotriene B4 appear in HL60 cells upon the induction of differentiation and also lend strong support to the use of these HL60 cells as a model of human myeloid differentiation. We have also characterized the emergence of a secretory response to fMet‐Leu‐Phe and leukotriene B4 in cytochalasin B treated HL60 cells. In addition, it is found that differentiation was required for the calcium ionophore A23187 to express its secretory activity toward the HL60 cells. This last set of results implies that differentiation is accompanied by the coordinated appearance of surface receptors and cytoplasmic factors required for the expression of cellular responsiveness.