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Behavioural studies of MHC‐congenic mice and rats have focused primarily on mate choice and the ability to discriminate between strains by their urine odours, but these strains may differ in other behaviours, such as activity and ultrasonic vocalizations. Ivanyi (1978, Proc. Roy. Soc. Lord. 202, 117–158) has reviewed the physiological differences associated with the MHC, many of which could influence behaviour. We have started a systematic study of behavioural development and adult behaviour in MHC‐congenic mice. A developmental test battery (growth, rate, locomotion, grooming, eye opening, ultrasonic vocalizations, etc.) was used to examine differences between C57BL/6J vs. B6‐H‐2bm1 and C57BL/10SnJ vs. B10.BR/sgSnJ mice. A test battery of spontaneous behaviours (activity, exploration, ultrasonic vocalizations, etc.) was used to examine behavioural differences between adult C57BL/6J vs. B6‐H‐2bm1 and C57BL/10SnJ vs. B10.BR/sgSnJ mice. Differences in development and in adult behaviours between these MHC-congenic strains is discussed in relation to possible neural, endocrine and immune system differences. Future studies will compare MHC-congenic mice on levels of anxiety, sociosexual behaviour and on learning paradigms.