Summary Lymphocytes from human peripheral blood were cultured in vitro. The distribution of antigenic determinants with specificity of immunoglobulin light or heavy chains on normal and phytohaemagglutinin (PHA) stimulated cells was examined by membrane immunofluorescence and immunoelectron microscopy. It appeared that a relatively large population of both normal and stimulated lymphocytes carried light chain determinants on their surface. Usually considerably more lymhocytes reacted for ϰ than for λ determinants. In contrast no or only few cells reacted with reagents specific for heavy chain (γ or μ) determinants. At the ultrastructural level, the reactivity to antilight chain antibodies in unstimulated cultures was seen to be confined to discontinuous areas of varying size on the surface of cells with the typical morphology of small to medium sized lymphocytes. In cultures treated with PHA, lymphocytes undergoing blastoid transformation often showed larger patches of antilight chain reactivity than unstimulated lymphocytes. The uropods of stimulated lymphocytes often showed a strong reactivity with antilight chain antibodies. No reactions were obtained with antibodies to heavy chains. In contrast, normal as well as PHA-stimulated lymphocytes stained for human species antigens by means of heterologous antisera from rabbits, exhibited an even distribution of the ferritin label over large areas of their surface.