An unfamiliar or strange situation was used as a setting to highlight individual differences in an infant's responses to brief separations from his mother and his reactions to a stranger. Two groups were observed, one composed of 23 white, middle class mother-infant pairs, the other of 33 pairs. Eight episodes, presented in fixed order for all pairs, were used in which a baby faced an unfamiliar environment and also a stranger, both when his mother was present and when she was absent. Detailed conclusions are drawn from the test situations for three groups of mother-infant pairs: (1) babies whose mothers were sensitive to their needs and with whom there was harmonious interaction; (2) babies whose mothers consistently rejected them and with whom there was an unharmonious relationship; and (3) babies whose mothers interfere and also ignore them, but in which maternal rejection is either moderate or masked. (MK)