The present study was conducted to compare the efficacy of five different reaction conditions on the guanidination of lysine in casein and to establish optimum lysine : 0-methylisourea (OMIU) for maximum guanidination of lysine in casein and soya bean meal. The results indicate that the presence of glycine-NaOH buffer is not required for guanidination of proteins at pH 10.5. A OMIU concentration of 0.4 M was found to be as effective as 0.6 M for guanidination. Both OMIU-hydrogen sulphate and free OMIU were equally effective reagents in terms of conversion of lysine to homoarginine. The use of OMIU-hydrogen sulphate for guanidination and the use of ethanol to recover guanidinated protein, however, resulted in the formation of crystalline sodium sulphate, a known purgative agent, in the guanidinated material, and therefore are not recommended if the guanidinated protein is to be used in animal trials. The molar ratio of lysine : OMIU required for efficient lysine conversion to homoarginine varied for different protein sources. Ratios required for maximum conversion for casein and soya bean meal were determined to be 1 : 10 and 1 : 16, respectively. A simple procedure developed for the large-scale guanidination
(5-10 kg batches) of proteins is also described. The results showed that guanidination of proteins can be easily scaled up from 20 g to 5-10 kg and that large-scale guanidination is feasible and efficient.