An electron microscopic examination was made of masses identified as altered heterochromatin in the cytoplasm of childhood leukaemic lymphoblasts, and their relation to 'virus-like' particles (VLP). The masses were found to occur more frequently in leukaemic lymphoblasts than in normal lymphocytes, but abnormally frequent occurrence was a feature of non T rather than T or B lymphoblasts. Although these masses may be derived by phagocytosis of dead cells or cellular material, a more likely origin is by detachment of altered portions of the containing cell's own nucleus. Such in situ alterations in nuclei were seen in leukaemic lymphoblasts but not in normal lymphocytes. A close relation demonstrated between altered chromatin and VLP might suggest that VLP are a response to the presence of damaged chromatin. However, the occurrence in some VLP of solid cores and budding might make it more likely that VLP are viral, with chromatin damage as a morphologically recognizable cytopathic effect of their activity.