This study was undertaken in order to evaluate the cytogenetic and immunological responses to the effective, harmless and world-wide used vaccines of I. rubella and II. measles-mumps-rubella (M-M-RII). In one group (A) of five girls vaccinated against rubella and in another group (B) of four boys and two girls vaccinated against measles-mumps-rubella, the following parameters were studied before and repeatedly after vaccination: (a) SCE frequency, in peripheral lymphocytes, (b) DNA-synthesis, in peripheral mononuclear cells, and (c) antibody titres. The mononuclear cell proliferation rate was elevated between the 3rd and 7th day, preceeding the humoral immunological reactions, which began after the 25th day (group A) and the 28th day (group B). The latter findings coincided with a significant increase of SCE frequency in group A (one child) and in group B (all six children); in no case did the highest SCE/cell ratio exceed the normal value.