Cytoid bodies represent ovoid, round or polygonal, approximately cellsized structures. In human skin such bodies may occur under normal as well as under pathological conditions. A number of heterogenous materials contribute to the morphogenesis of cytoid bodies, but in sections of human skin prepared for routine histology, different groups of cytoid bodies can not always be distinguished from each other. However, such a differentiation is necessary, if their presence is to be utilized as a diagnostic parameter or interpreted as a sign of physiological or pathological events in the dermoepidermal junction area. The object of the present studies was to (i) characterize the different groups of cytoid bodies by histological, histochemical, immunological and electron microscopical techniques, (ii) elucidate their nature, origin and morphogenesis and (iii) determine their significance in the histology of the skin. The following results were obtained: (1) Elastic globes can easily be identified by their bright autofluorescence and their affinity for elastin stains. Electron microscopically they exhibit a mixture of amorphous, granular and filamentous material, thus showing simlarities with elastic fibers. They are regularly found in normal skin of the extremities and the face but usually are absent on the trunk. Therefore their demonstration may be of importance in forensic medicine by allowing a better determination of the origin of isolated skin pieces. (2) Russel bodies may show gross variations in their histological, histochemical and ultrastructural properties. They contain different amounts of carbohydrates, lipids, proteins and immunoglobulins. These variable component result in a polymorphous structure. Russel bodies are produced by plasma cells and can frequently be found in skin infiltrates with a predominant admixture of this cell type. Their presence may be correlated with an increased local production of immunoglobulins, but their differentiation from other cytoid bodies and fungal elements is also of importance. (3) Civatte bodies are also eosinophil, PAS-positive and exhibit a typical fibrillar ultrastructure. They may be localised intraepidermally as well as in the upper corium. Although most frequently ecountered in lichen planus they may also be found in numerous other dermatoses and even in clinically normal skin. As indicated by their characteristics, they originate from epidermal keratinocytes and probably represent a morphological substrate of "apoptosis". (4) Amyloid is identified by the green birefringence of Congo red stained sections and by the more irregular arrangement of the individual filaments under the electron microscope. Cytoid aggregates of amyloid in the skin occur mainly in lichen amyloidosus and macular amyloidosis. Their presence should prompt further investigations of the patient for other signs of amyloidosis. (5) Fibrin in the form of cytoid clumps and clusters can frequently and specifically be demonstrated in skin diseases by immunofluorescence...