Three lots of Rocky Mt. spotted fever vaccine of the Spencer-Parker type (i.e.. prepd. from the tissues of infected adult Dermacentor andersoni), one manufactured in 1928 and 2 in 1929, and stored at 34[degree] to 40[degree] F, were retosted for potency in 1942. Each was found to be fully as potent as at the time of manufacture,, approx. 14, 12, and 12 years ago, respectively.