Initial stages of insulin-receptor interaction in two types of blood cells (erythrocytes and monocytes) as well as of thymosin-receptor interaction in thymic lymphocytes of rat were studied by electron microscope. Hormone-receptor complexes were marked by colloidal gold. There was general similarity at the initial stages of interaction of both peptide hormones and cellular, membrane receptors for them. Some characteristic and regular ultrastructural peculiarities were described in distribution, aggregation and early links of internalization of insulin-binding and thymosin-binding receptor sites, which could be taken into consideration analysing hormone-receptor interaction and of its defects at receptor and postreceptor level.