Twelve CF heterozygous and two CF homozygous women who were tested serially for the CF-lectin activities during one to two months of their menstrual cycles, were found consistently to have negative tests during menses. The specific hormonal alteration during menses that affects the assay is unknown. However, estrogenic medication administered without progesterone to postmenopausal heterozygous women also caused false-negative tests, suggesting that a balance between progesterone and estrogen is critical for the CF-lectin activity; mere depletion of these hormones alone does not interfere. The addition of estrogen in vitro also inhibited the CF-lectin activity at a physiologic concentration of 10(-6) M, and addition of an equivalent concentration of progesterone blocked this inhibitory effect. Future studies of the CF-lectin or its assay as a potential CF-carrier test must be limited to those women not menstruating at the time of blood drawing and not receiving estrogens. Hemolysis or contamination of the serum with RBC should invalidate that sample for testing, although the effect of RBC contamination can be circumvented with the addition of mannose. A more objective, simplified means of performing the CF-lectin assay is reported. A preliminary blind study taking these issues into consideration was 100 percent correct in detecting 16 CF patients and 17 obligate heterozygotes, and revealed 4 percent of 64 control subjects to have positive tests. The presence of this lectin-like factor may reflect the underlying biochemical defect responsible for this disease.