This study was designed to determine whether the cytocidal activity of immunotherapy such as cytotoxic peripheral blood lymphocytes (PBL), lymphokine-activated killer (LAK) cells, and chimeric anti-CD20 mouse/human monoclonal antibody, IDEC-C2B8, overcome vincristine (VCR) resistance in cultured cell lines derived from human leukemia/lymphoma. In addition, the relation between the susceptibility to these immunotherapies and the expression levels of HLA class 1 and ICAM-1 as well as CD20 on the cell surface was analyzed. Three of six VCR-resistant cell lines were less susceptible to PBL cytotoxicity compared with wild-type cells, whereas the susceptibility was kept in the other three VCR-resistant cell lines. Four of six VCR-resistant cell lines were less susceptible to LAK activity and the other two cell lines were as sensitive to LAK cells as their wild-type counterparts. There was no correlation between the susceptibility for PBL cytotoxicity and the expression of HLA class 1 in both wild and VCR-resistant cells. In contrast, ICAM-1 in the two cell lines that showed decreased susceptibility for LAK cytotoxicity disappeared, although that in one cell line increased. IDEC-C2B8 was effective only against B-cell lines expressing CD20. One cell line in which the expression of CD20 increased was nearly six times more sensitive to IDEC-C2B8 than wild type. Thus, we concluded that the resistance to VCR in some tumor cell lines is associated with modified susceptibility for immunotherapies by the different expression of target molecules from those of wild-type counterparts.