In order to research the effectiveness of proteasome activator subunit 2 in carp’s immunologic response，we cloned the cDNA of proteasome activator subunit 2 from peripheral blood leukocyte of carp. Moreover，we also researched the expression information of PA28β in carp’s peripheral blood leukocyte. The cDNA library of peripheral blood leucocytes which were isolated from carp ( Cyprinus carpio L) and stimulated with mitogen PHA and ConA was screened by a probe labelled with DIG. Using DD-RTPCR, the probe considered as the differential expression fragment which is partial sequence of proteasome activator subunit 2 was obtained. After two rounds of screening from 8 thousand recombinate phages，the positive clone was obtained. Sequence analysis indicates that it contains a insert sequence of 1 175 bp in length with an full ORF encoding 244 amino acids of proteasome activator subunit 2，including PA28β and partis α subunit motifs. The protein sequence showed significant homologues (95% identity) with zebrafish proteasome activator PA28β ，which has 244 amino acids. Multiple sequence alignment with other species showed that carp PA28β sequence has the highest homologue with that of zebrafish, and has the lowest homologue with Xenopus tropicalis，which is 55 % identity. Using the total RNA extracted from peripheral blood leucocyte which was isolated，cultured and stimulated with mitogen PHA and ConA，this paper did semi-quantitate RT-PCR. It displays that the quantity of PA28β in leucocyte stimulated with mitogen PHA and ConA is obviously larger than in normal carp leucocytes in the prophas(4 h). However，it is not always much than that of the normal leucocyte at the same time with the time going, the express of PA28β mRNA in carp leucocytes stimulated by PHA for 4 h is higher than stimulated for 12 h，and the express of PA28β mRNA stimulated by ConA for 4 h is higher than 24 h. This is the first report for full length cDNA sequence of the PA28β from carp. The GenBank accession number for the sequence is EU255233.