AbstractGlucosinolates and their highly reactive breakdown products (mainly isothiocyanates, ITC) belong to the group of very important natural toxicants. This paper describes the interactions of benzyl‐ITC with the egg‐white protein fractions ovalbumin, conalbumin, ovomucoid, and lysozyme and the resulting formation of protein‐ITC derivatives. Benzyl‐ITC reacts with the amino groups of the proteins, forming thiourea derivatives. Such reactions are characterised by decreases in the levels of free amino groups and available lysine. Other reaction sites of benzyl‐ITC are sulphydryi side‐chains of proteins, the results being formation of dithiocarbamate esters. This in turn results in a decrease of SH‐groups due to carbon disulphide being released from their esters. The present study describes the reactions of benzyl‐ITC with amino groups and sulphydryl side‐chains, and the subsequent decrease of lysine and crysteine after derivation. Changes in the behaviour of ITC‐derived egg‐white protein fractions were analysed by SDS‐PAGE and RP‐HPLC.