The expression of mismatch repair proteins hMSH2 and hMLH1 was investigated in human ovarian cancer cell lines and in biopsies of ovarian carcinomas obtained from 20 patients undergoing surgical operation. By Western blotting analysis hMSH2 protein was detected in all the tumor samples analyzed and in eight out of nine human ovarian cancer cell lines, while hMLH1 was undetectable in four out of 20 ovarian tumors and in five out of nine human ovarian cancer cell lines analyzed. The possible presence of frameshift mutations in the BAX gene, which contains a sequence of eight contiguous guanines in its third exon, was tested in all the samples. All the cell lines presented the normal alleles for the BAX gene while only in one of the tumor samples a heterozygous frameshift mutation was found. The frameshift mutation was associated to a low, almost undetectable, level of BAX protein which was instead present at much higher levels in all the other samples investigated. The results indicate that frameshift mutations in the BAX gene, possibly arising as a consequence of microsatellite instability (detectable in these tumors), is detectable in human ovarian cancer although quantitatively it does not appear to be a major determinant of the low apoptotic response to chemotherapy observed in ovarian cancer cells.