In an attempt to clarify the controversy about the distinction between Burkitt's and non-Burkitt's small noncleaved lymphomas, 55 cell lines derived from 48 Burkitt's lymphoma patients were characterized by morphometry on plastic-embedded sections. The results of the measurements permitted the identification of five main cytologic types, with regard to nuclear size, nuclear area dispersion and irregularity of nuclear profiles. The presence of the Epstein-Barr virus (EBV) and the geographic origin of the tumors seemed to play essential roles in the determination of nuclear size, with a significantly larger size seen in EBV-positive cell lines, and especially in the African lines among these. Immunoglobulin profile and monoclonal antibody expression also correlated with the nuclear size. Two conclusions may be drawn from this analysis. An in vivo transformation of the cells of Burkitt's lymphoma can be postulated to explain the wide morphologic spectrum of lymphomas presenting a rearrangement of chromosome 8. The fact that typical and atypical Burkitt's lymphomas cannot be differentiated by study of their derived cell lines raises the question as to the validity of the distinction between the two subtypes of small noncleaved lymphomas.