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The present work provides the first information concerning the chemoattractant activity of trout recombinant IL-1beta and its derived peptides, referred to as P1, P2 and P3. The predicted rainbow trout mature interleukin-1beta peptide was produced as a recombinant protein in Escherichia coli. The first peptide, P1, corresponded to fragment 146-157 (YVTPVPIETEAR) of the trout sequence and had an MW of 137 kDa. It was equivalent to a region known to be part of the receptor binding domain from the mammalian crystal structure of IL-1beta complexed to its receptor. P2 was used as control peptide, consisting of the same 12 amino acids as P1, but arranged in a random sequence (VVEEYIRAPPTT). P3 was synthesised to complex with an adjacent region of the IL-1 receptor, and corresponded to fragment 207-216 (YRRNTGVDIS) of the trout sequence, with an MW of 1.18 kDa. Migration was stimulated when leucocytes were exposed to concentrations of > or = 10 ng ml(-1) rIL-1beta. Peptide P3 also induced leucocyte migration, with an optimal dose of 0.25 mM being recorded. While P1 had no effect on cell migration when used alone, synergism was evident as a consequence of combining P1 with a suboptimal dose (0.01 mM) of P3. No synergism occurred when cells were exposed to a combination of P3 and the control peptide P2.