Peripheral blood smears of 20 male and 20 female tamarins were examined for the presence of the nuclear drumstick appendage in neutrophils. This chromatin body characteristic of female cells was found in significant numbers of neutrophils from males and in some females was found to be absent or in a lower frequency than expected. The distribution of drumstick-bearing neutrophils among these animals was such that they suggested male-female blood chimerism resulting from prenatal vascular placental anastomoses between fraternal twins of this species. Heterosexual chimerism was confirmed by sex-chromosome analyses of tissues from both male and female animals. The drumstick marker, however, did not provide a reliable quantitative index of chimerism as determined by sex-chromosome analysis. Chimerism was found in blood, bone-marrow, lymph-node, and splenic tissue indicating intermixing of the hematopoietic systems of fraternal twins. A new micromethod utilizing an in vivo diffusion chamber for the procurement of mitotic cells for chromosome spreads was described.