Radioimmunoassay, gel filtration, and immunoperoxidase methods were used to study the occurrence, properties, and concentration of pregnancy-associated plasma protein A (PAPP-A) in the human ovary and in the follicular fluid from 97 hyperstimulated follicles from 29 infertile women participating in an in vitro fertilization program. At the detection level of 15 micrograms/l, PAPP-A was found in 83 of 97 follicular fluids, the levels ranging from undetectable to 483 micrograms/l (median, 130 micrograms/l). In gel filtration, PAPP-A immunoreactivity of follicular fluid eluted in the same volume as placental PAPP-A, and the dose-response curves of follicular fluid PAPP-A and purified PAPP-A were parallel. The PAPP-A concentration was not affected by prior treatment with a protease inhibitor. Follicular fluid aspirates containing the ovum had a higher PAPP-A concentration than those in which no ovum was detected (P less than 0.01), whereas no difference was found in the PAPP-A concentrations between follicles yielding an ovum which was fertilized and cleaved and those yielding an unfertilizable oocyte. There was a correlation between PAPP-A and estradiol or progesterone concentrations, and between the PAPP-A concentration and the volume of follicular fluid aspirate. In hyperstimulated unruptured follicles, PAPP-A was localized in the luteinized granulosa and theca interna cells, but not if luteinization was not observed. Corpus luteum cells were also PAPP-A positive, whereas unstimulated Graafian follicles were negative. Our results indicate that PAPP-A appears in the hyperstimulated follicle shortly before ovulation and may thus play a part in the early events of human reproduction.