Albumin, IgG and complement C3c were analyzed by immunonephelometry in cervical mucus collected daily at midcycle. There was a statistically significant difference in the amount of mucus recovered on the day of the luteinizing hormone (LH) surge and on the following day. Several nadirs in protein concentration could be visualized in 8 out of 14 subjects, whereas in 6 subjects, no such changes in concentration were found. In terms of soluble protein concentrations and amounts there were no differences between cycle days. For changes in soluble protein concentrations and amounts, no systematic time relation to the LH peak could be found.