Although analysis of the humoral components of the external secretions has revealed the predominance of secretory immunoglobulin A (SIgA), trace amounts of serum-type IgA, IgG and IgM have also been detected (Tomasi, 1972). In the reproductive fluids, higher proportions of IgG have been observed (Acharya, Khambhla & Goodman, 1968; Hulka & Omran, 1969). The effec- tiveness of antibodies depends on whether or not they are stable in the environ- ment in which they are to function. Seminal plasma is known to possess proteo- lytic activity, a condition that is generally unfavourable for the survival of ordinary proteins (Katsh, Aguirre & Katsh, 1968; Suominen & Niemi, 1970), and antibodies may not therefore be able to survive and function in these fluids. However, SIgA is known to be resistant to proteolysis (Cederblad, Johansson & Rymo, 1966; Brown, Newcomb & Ishizaka, 1970). The present study was designed to evaluate the structural stability of various classes of immunoglobulin (IgG, IgA, SIgA and IgM) in the secretions of male and female reproductive systems, and to follow changes in the biological activity of antibodies under the same experimental conditions Semen samples were obtained from healthy adults by masturbation and were allowed to liquefy at room temperature for 15 to 30 min. After centrifugation, the seminal plasma (SMP) was separated from the sediment and stored at — 20°C. Cervical mucus was collected by aspiration from healthy adult women after insertion of a 15-cm long blunt-tipped pipette directly into the external cervical os. The secretion was suspended in 2 ml of 0-85% NaCl. The suspen¬ sion was mixed thoroughly, centrifuged, and the supernatant cervical fluid (CF) was removed and stored at — 70°C. The SMPs used in this study were found to have moderate proteolytic activity, while pooled CF with a protein content of 185-0 mg/100 ml had only weak activity (Charney & Tomarelli, 1947). The preparations of serum IgG, IgA and IgM and secretory IgA have been described (Sirisinha & Charupatana, 1970; Tornasi, Tan, Solomon & Prender- gast, 1965). Purified Igs were trace-labelled with 125I by the chloramine-T method (McConahey & Dixon, 1966) and the labelled protein was separated