Primary acute myeloid leukaemia (AML) blasts can be induced to differentiate into dendritic-like leukaemia cells (DLLC) by culture with certain cytokine combinations. DLLC offer potential for use as autologous vaccines based on their ability to present putative leukaemia-specific antigens to T cells. It has been reported, however, that in around 30-50% of AML cases the leukaemia cells are not capable of undergoing DLLC differentiation. The purpose of this study was to identify the features that represent successful DLLC differentiation and, for those cases shown to be resistant to cytokine-induced differentiation, to use differentiating agents in an attempt to overcome the differentiation block. Leukaemia cells derived from 42 patients with AML were cultured in vitro with cytokines GM-CSF, IL-4 and TNFalpha/CD40L. In 22 cases the leukaemic cells underwent DLLC differentiation based on characteristic morphological changes and expression of costimulatory and dendritic cell-associated molecules. Four cases were not evaluable because of poor viability over the culture period. The remaining 16 cases failed to show evidence of DLLC differentiation. Many of these differentiation resistant cases were associated with poor risk karyotypic features. Nine of the resistant cases were selected for further study. Differentiating agents trichostatin (TSA), azacytidine (AZA) and bryostatin (BRYO) were used in combination with cytokines for the first 96 h of the culture period. Bryostatin (BRYO) alone was shown to be capable of overcoming differentiation resistance and allowing DLLC differentiation to proceed.