Whether research on monkeys is ethical will depend on what you understand the nature of ethics to be. If you understand ethics as a consequentialist, it's unlikely you will think experimentation is unethical. If you understand ethics as a deontologist, then whether experimentation is ethical depends on your principles.
A consequentialist considers the consequences of experimentation. If the consequences are good, if they provide benefits for research in medicine that could not be provided otherwise, they are judged as a good thing. A deontologist simply argues from principles they think are most appropriate for judging whether experimentation on monkeys is ethical. For instance, a deontologist could extend the sanctity of human life to all animals, and judge that experimenting on animals devalues their dignity and is therefore wrong.