A practical and accurate method of determining ovulation using cervical mucus is described. Ovulatory cycles from 32 healthy women were assessed using hormonal and nonhormonal methods. Cervical mucus extractable immunoglobulins were assayed for the complete cycles of nine women and randomly collected from 23 additional women with normal cycles. All normal ovulatory women showed a minimum concentration present at midcycle when optimum conditions for fertilization existed. Using newer laser nephelometry techniques, the content of extractable immunoglobulin G and immunoglobulin A per sample weight confirmed the cyclic nature of the concentration of immunoglobulin in cervical mucus, and correlated well with previously defined biophysical properties of cervical mucus. Nephelometry proved to be rapid, accurate, and an inexpensive technique for determining ovulation and has potential for clinical use.