The present paper highlights the chief results of experimental studies recently carried out in the author's laboratory on the stimulatory effects of low level radiation on immunity. The intercellular relationship within the immune system after whole-body irradiation(WBI) with low doses of X- and gamma-rays was briefly outlined with the emphasis on the potentiating effect of low dose radiation (LDR) on the helper T cells which was thought to occupy a pivotal position in the process of immuno-stimulation. Data suggesting the facilitation by LDR of the signal transduction process within the T cells were presented. Some hypotheses prevailing in the literature for the explanation of the stimulatory effect of LDR were critically evaluated. Existing experimental data both from the literature and from the author's. laboratory were analysed giving no strong support to the speculation of either the preferential deletion of the radiosensitive suppressor T subset or the increased apoptosis of the T cell "precursors" in the thymus leading to secondary immunologic augmentation by low doses which stimulate immune functions. The possible influence of systemic changes, especially those in the neuroendocrine regulation, on the T cell potentiation following LDR was suggested, Finally the biological implications of the hormetic effects of LDR were discussed.