Other authors have demonstrated earlier that cells of normal synovium contain metallothionein. The protein was also detected in several other normal cell types and in tumors derived from the cells. Metallothionein content is thought to reflect proliferative activity of neoplastic cells. Therefore, it was decided to demonstrate metallothionein expression in various types of synovial sarcoma. The present study aimed to determine metallothionein cellular expression by immunocytochemical techniques in nine cases of biphasic, six cases of monophasic (spindle cell), and five cases of poorly differentiated synovial sarcoma, and to compare the expression with those of vimentin and cytokeratin 19. Metallothionein expression was demonstrated in epithelioid cells in all cases of biphasic type sarcoma and in spindle cells in all cases of monophasic type tumors. In poorly differentiated tumors, metallothionein expression was detected in four of five cases (80%). Expression of cytokeratin 19 was typical for epithelioid cells and expression of vimentin for spindle cells of synovial sarcoma. A much less pronounced expression of the proteins was observed in poorly differentiated tumors. The results indicate that metallothionein expression may prove useful in differential diagnosis and for defining prognosis in cases of synovial sarcomas.