SCIENTIFIC ABSTRACT Large amounts of cold nuclearly processed gas (primarily oxygen and iron) are expected to be present in the inner unshocked regions of young supernova remnants (SNRs). It is proposed to detect and establish characteristics of this material by means of fine-structure lines. About 20 targets are considered, some of which being observed at several positions. Determining the mass and chemical composition of supernova ejecta is crucial for three theories: stellar evolution, supernova explosions, and galactic evolution. This is a priori best performed in young SNRs in which mixing with the ISM is not yet achieved. Current information from X-ray, UV, and optical studies is scarce and far-IR spectroscopy appears as a valuable alternative. Information about the dominant elements present in the inner part of SNRs (e.g., O, Ne, Si, S, Fe, and Ni) will provide a basis to connect SNRs to the events from which they were born. SN1987a is clearly a special target. OBSERVATION SUMMARY The grating mode of ISO/LWS is well-suited to this programme since overall velocity fields of a few thousand km/s are typical. Wavelength oversampling should provide useful information on velocities in bright objects. Full grating spectra are necessary in most cases because SNRs are complex objects. The probability of "unexpected" spectral features and of projection of different components of the SNR in the observation lobe is large. The targets should include all classical young SNRs because they are all different and even their progenitors may be controversial. In a few cases several positions should be observed to study composition variations and to allow subtraction of possible "foreground" emission from, e.g., the outermost shocked ISM. This "foreground" emission is evidently most interesting in studying other aspects of SNRs (interstellar shocks, dust heating, molecules...) not explicitly considered here. This "multipurpose" aspect of any spectroscopic programme about young SNRs should be kept in mind. Since the proposal mainly concerns detection of emission lines of ionic species not expected in the background emission, no "off-source" positions are considered.