1. Semi-synthetic diets, with dried microbial cells (Pruteen) as the nitrogen source, were used to measure N retention in 50 kg pigs which had been given only sufficient N (5.3 g/d) to maintain N equilibrium for the previous 12 d. Control pigs were given 33.2 gN/d. 2. Metabolic faecal N losses were 1.62 g/d (1.2 g/kg dry matter eaten) and endogenous urinary losses were 3.90 g/d. 3. Realimentation of N-deprived pigs was achieved with diets providing 20.0, 33.2 and 67.4 g N/d and daily rates for N retention were 11.2, 17.8 and 25.9 respectively; the corresponding value for control pigs was 15.0 g. 'Catch-up' protein growth was demonstrated in pigs given both 33.2 and 67.4 g N/d. In the former instance, this was associated with an increase in the efficiency of utilization of dietary N. 4. The biological value of the protein in Pruteen was 0.85, and it appeared that under conditions of increased demand for N the pig could utilize some of the nucleic acid-N fraction of this protein source.