Suppression subtractive hybridisation (SSH) is a powerful means to identify genes of cytokines and other genes expressing small amount of mRNA, as the SSH utilises PCR for the amplification of the subtracted and equalised gene fragments (Diatchenko et al., 1996). By using SSH, full-length cDNAs encoding carp cytokines homologous to mammalian CC chemokine (S-84), allograft inflammatory factor-1 (L-68), natural killer cell enhancing factor (L-128) and interleukin-1β (C-44)(Fujiki et al., 1999, 2000) have been cloned. In the SSH between carp leucocytes collected before and after dynamic migration elicited by sodium alginate (Fujiki et al., 1999), the generated cDNA fragments encoded some cytokines including one similar to human pre-B cell enhancing factor (PBEF), which remained to be fully sequenced and characterised. In the present work, using this fragment as a probe, a full-length clone named S-63 was isolated from the ZAP Express cDNA library (Stratagene) as previously described (Fujiki et al., 2000), and its expression in vivo was evaluated by RT-PCR. Clone S-63 is 2051 bp-long including 5 and 3 untranslated regions and the entire coding region for 493 amino acids. The coding region is 72· 3 and 86· 2% identical to that of human PBEF (GenBank accession No. P43490) at the nucleotide level (data not shown) and the amino acid level (Fig. 1), respectively. This strongly indicates that S-63 is a carp homologue of human PBEF. Like human PBEF, carp PBEF lacks a signal sequence as determined by the SignalP program (Nielsen et al., 1997), though human PBEF is indicated to be a secreted protein. Two N-glycosylation sites present in human PBEF are conserved in the carp homologue. Carp PBEF has five cysteines, one less than human PBEF. The isoelectric point calculated by the Compute pI/Mw program (Wilkins et al., 1998) is 6· 27, similar to 6· 69 of human PBEF. To examine if carp PBEF is upregulated in response to immunopotentiators in vivo, RT-PCR was performed on PBEF mRNA in head kidney cells (HKC) and peritoneal cells (PC) of carp which had been injected with sodium alginate (SA, 2 mg 100 g 1 body weight) and scleroglucan (SG, 1 mg 100 g 1 body weight) 48 h earlier, as previously described (Fujiki et al., 2000). Oligonucleotides used as PCR primers for carp PBEF (327 bp) were: sense primer 5-CCATCGAGATCAAGGCTGTG (369–388), and antisense