Human blood samples from adults and newborns of blood groups O, A, and B were treated with either anti-A blood grouping serum, ferritin-conjugated anti-A serum, free ferritin, or saline and then prepared for electron microscopy. Morphological differences were observed between the untreated erythrocytes of infants and adults. Circulating red cells of newborns were frequently vesiculated (25.5%), whereas those of adults only occasionally showed vesicles (5.5%). On the basis of morphology and incidence, the majority of these vesiculated cells seemed to be mature erythrocytes. The introduction of anti-A serum to group A erythrocytes of infants appeared to stimulate vesicle formation, but anti-A serum did not have a similar effect on group O or B cells of infants or on group A cells of adults. Vesicles which formed in response to antiserum treatment appeared to be the result of pinocytosis. In contrast to the well dispersed ferritin along the membrane of agglutinated adult cells, the ferritin particles on the infants' cells were frequently clustered at irregular intervals. These accumulations seemed to lead to invaginations of the cell membrane, resulting in ferritin-lined intracytoplasmic vesicles. The addition of free ferritin or ferritin-conjugated antibodies of the wrong specificity to red cells did not increase vesicle formation.