Abstract Lupin seeds produced from two new varieties (CH304/70, CH304/73) of UK-grown autumn-sown determinate genotypes were studied for their chemical composition including fatty and amino acid contents together with the rumen degradation of nitrogen and the post-ruminal availability of undegraded nitrogen. A soya bean meal (SBM) was included in the rumen degradation work for comparison. In addition, for one variety (CH304/70), digestibility and energy values were measured using sheep and a comparison with SBM as a protein supplement for young growing bulls was made. The crude protein contents of varieties CH304/70 and 73 were 362 and 305 g kg −1 dry matter (DM), respectively, and their chemical composition was in general similar to previous reports. Rumen degradability of nitrogen (at assumed rumen outflow rate of 0.05 h −1) was higher than that of SBM (650, 610 and 550 g kg −1 feed nitrogen for CH304/70, CH304/73 and SBM, respectively) but considerably lower than most other reports largely because of much lower nitrogen solubilities. This was possibly due to the relatively coarse milling used in the present work. Increasing the proportion of CH304/70 in the diets of wether sheep from zero to 0.4 produced linear responses in the digestibility of all fractions except lipid, and led to an estimated metabolisable energy value for the lupin seeds alone of 16.9 MJ kg −1 DM. When SBM in the diets of growing bulls (liveweight ∼ 180 kg) was replaced on an isonitrogenous basis by lupin seeds no significant effect on performance was seen.