The results of a comparative study of the efficacy and acceptability of 15(S)15-methyl prostaglandin F2alpha (15-Me-PGF2alpha) administered as a single i.m. injection or vaginal suppositories (15-Me-PGF2alpha methyl ester) every 3rd hr for termination of very early human pregnancy is reported. The amenorrhoic period varied from 37 to 60 days. Group I (30 cases) received 0.6 mg as a single i.m. injection without any pretreatment. Retrospectively 24 of the 30 women were in fact pregnant and 22 of them aborted. Group II received suppositories (1.0 or 1.5 mg per suppository). In this group all women were pregnant and they all aborted. Symptoms such as pain, bleeding, vomiting and diarrhea started in general earlier in the i.m. group and they were more marked. In the present series the efficacy and acceptability were highest for the vaginal route of administration.