Alfaxalone is a general anesthetic used widely in veterinary medicine and has a chemical structure similar to progesterone. Here we report an unexpectedly high serum progesterone value in a female common marmoset following the treatment with alfaxalone. Cross-reactivity of the antibody used in the progesterone assay with alfaxalone most likely explains this finding, and therefore evaluation of serum progesterone results in animals treated with alfaxalone should be done with caution.