In order to investigate the effects of penicillin treatment in the course of syphilis, I have made observations on them from various standpoints in 75 cases of syphilis of every stadium, and obtained the following results: 1) By taking the average, spirochaeta pallida disappeared in 2.1 days, took 3.6 days for the granulation tissue to become fresh, 9.3 days to cure the chancre and 14 days for the induration to, disappear. Moreover, roseola disappeared in 9.3 days, papule in 7.2 days and in 6 to 10 days it showed an excellent effect on gumma. 2) The time needed for the disappearance of eruption depended on the amount of penicillin injected daily. 3) By examining the transition of the serum reaction in each stadium of syphilis, penicillin showed an exccllent effect on primary syphilis, and both in recent and latent secondary syphilis 10 cases of 14 turned negative or diminished in degrees, but in the late syphilis the effect of penicillin was not reliable. 4) Penicillin was also eflective on the so called anti therapeutic. syphilis in latent tertiary syphilis. syphilitie myelitis and congenital syphilis. 5) 5,400,000 to 6,000,000 units of penicillin as a total was the most effective quantity for penicillin treatment. 6) Injecting 600,000 units of penicillin at once every day was the most effective application. 7) 13 cases out of 75 showed slight reation due to the penicillin injection. 8) These reations tended to appear at the initial stage of penicillin treatment. 9) 84.7% of the patients lost more or less weight during the treatment, especially the late syphilis cases, who lost more weight, and also, they were be found more frequeutly in the cases which showed improvement in the serum reation. 10) Albumin, glycose and urobilin did not appear in the urine during the treatment. 11) The increase of the daily urine quanity was found in the recent and latent secondary syphilis and the latent tertiary syphilis. 12) Penicillin improved the occult insufficiency of the liver function caused by syphilis. 13) No cases showed insufficiency of the liver function due to the penicillin treatment. 14) The general symtom caused by syphlilis took a favourable turn with penicillin trcatment.