A review of the literature yielded no scientific basis for current recommendations on the length of time spermicide-filled cervical caps can be worn while continuing to protect against pregnancy. Ten cervical cap users provided 99 samples of cap contents after a variety of insertion periods using Gynol II (R), Ortho Crerne (R) and vehicle controls. Testing for spermicidal effect was performed in the laboratory using quality semen from two donors and employing the Multiple Exposure Photography system of semen analysis. Criteria for determining spermicide effectiveness were developed. Results suggest that 3 to 4 days is the reasonable limit for cap use with Gynol II (R) and 5 days with Ortho Creme (R), contrasted with 1 day limits for Gynol II (R) and 2 day limits for Ortho Creme (R) using existing criteria. Limited experience with samples exposed to semen in vivo suggest that spermicidal potency is not lost through repeated intercourse. Recommendations for validating these limits with clinical studies are made.