Toxicological research for humans requires model animals that are physiologically and/or developmentally closely related to one another. The development of the immune system (IS) in the cynomolgus monkey (Macaca fascicularis) was assessed and compared with selected data of humans and mice with the aim of contributing arguments to the discussion of the most appropriate animal model in toxicological research. Details of the developing IS in the cynomolgus monkey from embryonic day 35 to birth are investigated utilizing cluster development (CD) antibodies. Early immunoreactivity with CD 68, CD 117, and HLA-DR antibody is apparent from days 40 and 45 onward. All principal cell lines-T-, B-, and NK-cells-are present on day 100 in both thymus and peripheral organs. We discuss investigations of the cynomolgus monkey IS development with the reported development in humans and mice and stress 4 topics of significant interspecies differences to be considered in the decision for the appropriate animal model in a toxicological study.