The effect of lettuce cultivars on the nitrosation rate of proline was investigated. The lettuce was analysed for the presence of phenolic compounds. Lettuce and/or fish was incubated with nitrite under acidic conditions, and the incubation mixtures were investigated for the presence of N-nitroso compounds and mutagenic activity. Both volatile N-nitrosamines and mutagenic nonvolatile N-nitrosamines were detected. The formation of mutagenic N-nitroso compounds was also studied in selected cheese products after treatment with nitrite under acidic conditions. No direct relationship was observed between the total N-nitroso content of the samples and the corresponding mutagenicity. The ability of cheese to inhibit the direct mutagenicity occurring in fava beans after treatment with nitrite under acidic conditions was investigated. The antimutagenic factor, possibly casein, in cheese was not extractable with different solvents.