The degree of heterogeneity among subtypes was evaluated in 39 of the most frequent malignant, diffusely growing small B cell lymphomas by a combination of morphometry, automated image analysis, and immunocytologic techniques. Cluster analysis of nuclear profile parameters, including nuclear area, circularity factor, and chromatin distribution pattern, distinguished 3 groups. Each group was characterized by the preponderance of certain nuclear profile types, i.e. (a) small, rather regular (roundish) and dark staining (typical small lymphocytic lymphomas and immunocytomas); (b) small to intermediate size, rather regular (roundish) and pale staining (small to intermediate size variant); or (c) small to intermediate size, irregular, rather pale staining (diffuse follicular small cleaved cell (centrocytic) lymphomas and, probably, polymorphous immunocytomas). Group 3 was clearly distinct from the rest, but groups 1 and 2 also differed significantly. Every case displayed a mixture of the 13 registered nuclear profile types. Lymphoid cells with similar nuclear profile features occurred in B cell zones of non-neoplastic lymph nodes. Frequency distribution of nuclear profile parameters significantly differed from one lymphoma group to another. However, there were considerable overlaps. No clear correlation was found between nuclear profile types and immunophenotypes. Cellular surface antigen patterns showed an extensive intra- and inter-case variability. The findings support the notion of a certain individuality of malignant B cell lymphomas and of a marked heterogeneity among their subtypes.