Samples of 5 relatively pure crystalline penicillin fractions have been tested for rickettsiostatic activity in typhus-infected embryonate eggs. Penicillin X appeared to be about twice as effective on a gravimetric basis, and about 4 times as effective on a (S. aureus) unitage basis as penicillin G. Penicillin F, di-hydro F, and K were much less effective. On a (S. aureus) unitage basis, penicillin X showed a potency approximately double that of samples of crude penicillin used in 1944. Penicillin G showed a very much higher potency (on a unitage basis) than most samples of partly purified penicillin which we tested in 1945 and 1946. The differential rickettsiostatic potency of these 5 penicillins is roughly parallel to their differential activity against certain bacteria in vivo as determined by other, but differs from that reported in experimental spirochaetal infections, in which penicillin G has been found most effective.