The growth of lymphocyte colonies in a semisolid medium represents a proper studying model for lymphocytic proliferation and differentiation induced by an environmental signal. Human lymphocytes cultivation in agar enabled a continuous monitoring and quantitative expression of the ability of some lymphocytes to produce colonies (clonal expansion); it is not always proportionately bound to 3H thymidine uptake of the whole lymphocytic culture in a liquid medium. A preliminary evaluation showed the production of lymphocytic colonies in a semisolid medium as one of the cellular immunity markers. The results of the cultivation were compared with long-term liquid medium tissue cultures of four B lines derived from a Burkitt malignant lymphoma and two T lines from an acute T leukaemia: Cells of B lines retained the morphological features of the original tumour, T lines cells produced more varied nuclear and cellular shapes, and the PAS-positive glycogen globules were often found in their cytoplasm. There were ultrastructural differences especially obvious in the early growth of the different passages.