After Christian's hypothesis of a physiological regulation of population numbers, we want to analyze serum gamma‐globulin (immunoglobulin) levels in the common vole from populations of different density. A five‐year study was carried out on voles caught in spring, summer (early‐late), and autumn, in alfalfa fields of northern (low population density) and southern (high density) Poland. The lowest concentration of gamma‐globulins occurred in early summer and the highest in late summer and autumn, especially in a year of the highest density of voles. Sexually nonactive females showed the highest level of gamma‐globulin in late summer especially in less dence northern populations. The increase in the gamma‐globulin levels with age was significant in summer and in autumn (South and North) and more rapid in nonactive females than in nonactive males. The parasitism in comparison to gamma‐globulin level showed the negative correlation in summer and the positive one ‐ in autumn, that suggests the mutual dependence of parasitism and immune response in voles. The pattern of gamma‐globulin level changes as well as the variability of the parameter similar each year in sexually active males and different in females suggest a greater role of females in forming the long‐term cycle of immunological events. The greater variability and maximum level of the fraction in late summer preceded high density in autumn. The seasonal increase in population number as well as greater density on the South might be caused by an intensity of immunologic processes and simultanously higher viability and smaller fertility of vole. It suggests that immunological competency may be a factor regulating the numbers of common voles in natural populations.