A Rainbow trout Oncorhynchus mykiss (L.) clone encoding the low molecularweight proteasome LMP 2 has been cloned and sequenced from rainbow troutleukocyte cells that had been stimulated with the mitogen phytohemagglutinin(PHA). The cDNA was 1254 bp and had an open reading frame of 648 bpencoding 217 amino acids. The sequence has a high degree of identity withother published LMP 2 sequences varying from 84·4% identity with medaka(Oryzias latipes) to 61·4% identity with Xenopus laevis. Northern blot analysisshowed the LMP 2 mRNA to be expressed in a wide range of tissues, with twotranscripts in gill, spleen and anterior kidney RNA. Up regulation of the LMP2 mRNA transcript was shown in cells that were maintained in primaryculture following stimulation with the PHA mitogen for 24 h. A 3·7-foldincrease in mRNA was observed between control and stimulated cells.