A complete sample of 29 galaxies, selected from the IRAS Point Source Catalogue at a limiting flux of 2 Jy at 60 micron in the sky region delimited by the equatorial coordinates 21h < alpha < 5h and -22.5deg < delta < -26.5deg, has been already observed from the ground in the continuum at 1.3mm, spectroscopically at 115 GHz (CO (1-0)) and at 21 cm. With such an amount of FIR-mm information, it is to our knowledge the best available infrared sample for an ISO investigation. We propose to use PHOT and LWS to take FIR photometry and spectra of CII emission line for all the objects of the sample to address the following issues: (1) the total amount of dust present in the interstellar medium (ISM), (2) the global star formation activity and the relative contributions of the molecular and atomic phases in the formation of the CII line, (3) the luminosity function and evolution of IR galaxies. In order to constrain the dust properties the entire spectrum from the IR down to 1mm should be taken. Indeed, there is evidence from 1 mm data that a large amount of dust lies at cold temperatures so that IRAS data are not sufficient to correctly estimate it. The current uncertainties in the broad-band FIR spectrum of galaxies seriously affect, in particular, our knowledge of: (a) the spatial distribution of dust in galaxy discs; (b) the total amount of dust and the gas-to-dust ratio; (c) the effect of dust on the extinction of the optical light. A comparison between CO and HI data and spectra of CII will allow to probe the photon-dominated regions associated with massive star formation and the diffuse atomic phase of the interstellar medium. The FIR/submm data for such a complete sample will also allow us to estimate the luminosity functions of galaxies in all the FIR domain and better constrain their contribution to the background radiation. PHOT and LWS offer a unique possibility to complement the IRAS/mm, CO and HI data on our galaxy sample, hence allowing a very significantly improvement in our knowledge of the physical processes occuring inside them.