A method of studying boar seminal gel has been devised which enables investigation to be made on the chemical and physical properties of the gel formed in vitro from the bulbo-urethral (Cowper's) gland mucin and either vesicular secretion or seminal plasma. The majority of the haemagglutinin (protein H) in boar vesicular secretion combines with the mucin to form the gel. The content of Zn, Mg and citrate of the natural gel is less (often 0·9 × ) than that of the seminal plasma; this ratio also applies to the Kjeldahl N material imbibed in vitro from seminal plasma or vesicular secretion (as distinct from the N contributed by the mucin). The imbibed aqueous solution of seminal plasma or vesicular secretion can be defined as `interstitial' in that its components are diffusible from the gel matrix into buffer. Part of the interstitial water (possibly as much as 10%) is `bound' physicochemically to the gel matrix and is unavailable for solvation of the seminal fluid components.