A major focus for improving the diets in many less developed countries (LDCS) is the provision of rumen fermentable nitrogen (N) using protein supplements to complement N-deficient foods. However, in vitro digestibility methods usually use N-rich environments for the degradation of single foods. This conventional approach may give data which do not reflect the nutritive value of the N-deficient diets often on offer in LDCS, neither is it appropriate for using in vitro gas production to study protein supplementation. Our earlier study indicated that, by using a N-free medium, the gas production technique responded to added ammonium sulphate and urea. The ADAS standardized methodology, which used 10 ml of inoculum instead of the 5 ml used in the earlier study, was found not to be very responsive to N supplementation. The ADAS methodology was therefore investigated in order to develop a modified protocol for fermenting foods in an N-limited environment. The study involved using inocula diluted to different extents in N-free medium for fermenting N-deficient substrates in N-free and N-rich media. The modified protocol was then used for investigating the interactions between N-rich and N-deficient foods from north-west India.