The immunohistological reactivity of six retinoblastomas was investigated by means of 18 monoclonal antibodies and compared with that of adult and fetal retina. The antigenic profiles were found to be characteristic for each cell type studied and indicated that a panel of monoclonal antibodies could achieve a specific immunolocalisation not afforded by any single antibody. Immunohistological comparison between retinoblastoma and adult and fetal retinal cells provided evidence of the histogenesis of the tumour. The data suggest that the tumour arises from an early multipotential cell, which retains the capacity to develop differentiation characteristics associated with inner or outer retinal cell types, resulting in a heterogeneous tumour cell population. A cell with such differentiation potential predominates in the retina prior to the primitive neuroepithelial layer division at eight weeks' gestation.