A seal plug for conduits is disclosed having a plug core including at least one radially-expansible seal member for sealing the conduit in which it is disposed, while permitting passage of cables or the like therethrough. Until required for the cable or cables, the passages in the seal plug are closed by removable pin inserts, retainer means being provided to hold the inserts in place. The plug includes longitudinal compression means to bring the seal member into sealing contact with the conduit wall. Each passage may open to the circumference of the respective seal member via slits to permit sideways insertion of the cable therein.