Chemotaxis of guinea pig neutrophils induced by icaria chemotactic peptide (I-CP) and its analogue [Lys⁷]I-CP was suppressed by pretreatment of the cells with pertussis toxin (IAP) or removal of extra cellular Ca²⁺. O₂^- production by formylmethionyl-leucyl-phenylalanine (FMLP) or I-CP was also suppressed under those conditions. In order to investigate the internal signal given by these peptides, we measured intracellular Ca²⁺ changes both in neutrophil suspension and in single cells using confocal laser scanning microscopy. Both of these peptides and FMLP induced biphasic increase of intracellular calcium concentration ([Ca²⁺]_i. When neutrophils were pretreated with IAP and subsequently stimulated with I-CP or FMLP, the first peak of [Ca²⁺]_i increase was suppressed while the second peak remained. On the other hand, the [Ca²⁺]_i increase induced by [Lys⁷]I-CP was almost blocked by IAP treatment. These results suggest that the signal transduction pathway taken by [Lys⁷]I-CP is different from that of I-CP or FMLP.