The purpose of this work has been to obtain quantitative data for analysis of the chimeric state of mouse parabionts during the acute stage of parabiotic disease. A strain of mice carrying T6 marker chromosomes was used, permitting identification of donor and host cells in both parabiotic partners by cytological analysis. Two parabiotic combinations were studied: C57BL parabiosed to CBA-T6T6, and A parabiosed to CBA-T6T6. In the first combination, the C57BL partner fell ill; in the second, the A parabiont. Cytological analyses of the spleens and lymph nodes performed at the same time in both partners revealed striking asymmetrical chimerism between the sick animal and its healthy partner. Appreciable amounts of donor cells were identified in the spleens of animals dying of parabiotic disease, in contrast to the almost complete absence of foreign cells in their partners. A hypothetical mechanism of parabiotic disease is suggested to be mutual interaction between donor and host cell populations causing massive destruction of lymphoid tissue with consequent lymphoid atrophy and eventual death.