Due to the experimental importance of being able to assess cholesterol absorption during the study of atherogenesis in primates, the plasma isotope ratio technique developed for rats by Zilversmit (7) was tested in New and Old World monkeys and compared with a more cumbersome procedure involving fecal collection and analysis. Although fecal analysis demonstrated an unaccountable loss of the intubated isotope, the comparability of the plasma ratios to absorption calculated by other methods suggest that this techique is probably as reliable as any presently available. The loss of isotope may reflect the absence of dietary cholesterol.