Nonsteroidal biochemical markers of oocyte maturity could presumably aid in oocyte selection and in the timing of insemination for in vitro fertilization. We assessed the usefulness of six potential markers found in follicular fluid. We used kinetic immunonephelometry to measure concentrations of alpha 1-antitrypsin, alpha 2-macroglobulin, antithrombin III, ceruloplasmin, fibrinogen, and plasminogen in 53 fluids and the corresponding plasma from 20 women undergoing in vitro fertilization. Specimens were obtained from both mature and immature follicles from each woman. The respective protein concentrations in follicular fluid could be assayed with a between-assay CV of 1.5% to 3.2%. Analytical-recovery studies indicated that only fibrinogen and antithrombin III concentrations were altered (by 120% and 75%, respectively) during aspiration of the follicle into saline. All protein concentrations were significantly increased in mature follicles, as were the follicular fluid:plasma concentration ratios for alpha 1-antitrypsin, antithrombin III, and ceruloplasmin.