The pathology of lymphoid organs in 38 low birth weight (LBW) human infants has been evaluated by morphological and morphometric features. The gestational ages of infants ranged from 22 to 32 wks and their age at death varied from 1 hr to 153 days post partum. Infants were divided into 3 groups: 1) without antigenic effects, 2) with mild (bronchopneumonia), and 3) with severe antigenic effects, mainly sepsis. In mildly affected LBW infants, the fetal type of the immune reaction was found. It continued during the period studied (till 5 mths) and was manifested in the reaction of macrophages and the transformation of lymphocytes to lymphoblasts. Reactive centres of follicles and mature plasmocytes were not found. During the first months of postnatal development, an increase in the amount of lymphocytes in the lymphoid organs and in the rate of proliferation of reticular epithelium and a decrease in the area of the cortex in the thymus were found in all infants. In severely affected infants, the number and the size of follicles in the spleen decreased significantly and the total number of cells decreased more than 3 times. Similar changes were found in lymph nodes. These changes as well as the weak reaction of the thymus are the main features of the insufficiency and fast devastation of the lymphoid system. A compensatory increase in the number of neutrophils and eosinophils in the red pulp of the spleen and lymph nodes was found after the second week.