Inactivation of the retinoblastoma growth suppressor gene (RB) is responsible for the development of retinoblastomas and occurs frequently in osteosarcomas and small cell lung carcinoma. Knowledge about the involvement of RB in the pathogenesis of myeloid leukaemias is still scarce. In this study we looked at the expression of the retinoblastoma gene product (p105) in 20 primary myelomonocytic and monoblastic leukaemias by Western blotting and immunocytochemistry using the anti-p105-monoclonal antibody PMG3-245. We found absence of or barely detectable levels of p105 in 11 patients (55%). Absence of or low levels of p105 were correlated with a higher leucocyte count at presentation (133 x 10(9)/l v 83 x 10(9)/l) and with the occurrence of extramedullary leukaemia (8/10 v 2/10). We conclude that abnormal expression of RB with absence of p105 or strongly reduced p105 levels occurs frequently in myelomonocytic and monoblastic leukaemias and that this may be correlated with a more malignant course of the disease.