Biologically active fragments from polymorphonuclear leukocytes (PMN) are simplified systems that can be used to elucidate specific pathways by which cell function is altered. In the current study we have found that cytokineplasts, which are motile fragments derived from the leading front (protopod, lamellipodium) of human PMN, rapidly increase their intracellular free calcium concentration when stimulated by chemotactic formyl peptide or by leukotriene B4, as measured by Quin-2 acetoxymethyl ester fluorescence. As in the parent cell, extracellular EGTA blunts this response only partially. Hence, cytokineplasts retain a mobilizable internal calcium pool, despite a general lack of intracellular organelles. In addition, formyl peptide more than doubles the amount of cytoskeleton-associated (polymerized) actin. In contrast, cytoplasts made by high-speed, discontinuous gradient centrifugation of cytochalasin B-treated leukocytes also increase their intracellular free calcium on stimulation, but cytoskeleton-associated actin increases by only approximately 14%. Thus, defective motile function in the latter cytoplast is associated with compromised effector function (actin polymerization).