Calpain (EC 3.4.22.17; Ca2(+)-dependent cysteine endopeptidase) is known to exist in two forms of isozyme. Calpain I requires low (or microM)-Ca2+ for activation and calpain II requires high (or mM)-Ca2+. Both isozymes consist of one heavy (approx.80 kDa) and one light (approx. 30 kDa) subunit each. The heavy subunits of isozymes I and II are different genetic products, while the light subunits are identical. Antibodies respectively specific for the heavy subunits of pig calpains I and II were raised in rabbits, and the affinity-purified IgG proteins were used for Western blot analysis. When 23 human hematopoietic system cells were examined for the degree of their expression of the genes for calpains I and II, all of them were found to contain calpain I of detectable amounts in their cytosolic fluid. By contrast, only nine cell-line cells were positive in calpain II, and they were, without exception, the lineage which had been infected with HTLV-I, the retrovirus responsible for human adult T-cell leukemia. The enhanced production of calpain II in HTLV-I infected T-cells was also confirmed by running chromatographic analyses on the homogenates of these cells, and comparing them with those of uninfected T-cells. When YT-C3 cell, which is an uninfected, natural killer-like cell, was transfected with HTLV-I gene, the resulting transformed stable cells, YT-4 and YT-5.1, were found to produce increased amounts of calpain II concomitant with that of interleukin (IL)-2 receptor protein. These results suggest that the gene expression for calpain isozymes may vary during the course of differentiation of T-lymphocytes. The mechanism of regulation of calpain isozyme genes and the biological significance of the variation in expression during differentiation still remain unanswered.