To investigate the role of constitutive hsp70 in protein folding and to probe the supplementation by other hsps in this folding, yeast cells expressing reduced constitutive hsp70 proteins, ssa1ssa2, were transformed with a plasmid expressing a bacterial luciferase protein. With several independent clone cells of transformants, the levels of luciferase activity and some hsps, such as hsp104, hsp90, hsp70 and hsp26, were examined. The luciferase activity was significantly lower in ssa1ssa2 transformants than in the wild type (wt) cell transformed with the same plasmid. Among several clone cells of ssa1ssa2, the cells with higher luciferase activities exhibited higher amounts of Ssa4 which is known to be expressed instead of lacking Ssa1 and Ssa2. The luciferase activity closely correlated with the amount of Ssa proteins, more than with the amount of other hsps. It is suggested that constitutional Ssa "chaperones" are needed for the folding of proteins and, in cells lacking Ssa1 and Ssa2, the increased Ssa4 is thought to partly compensate for their role in the folding of luciferase in vivo.