The purpose of this study was to clarify the mother-infant interaction process by the lag sequential analysis. The examination of the lag sequential relation between mother and infant behaviors was carried out using conditional probability. To compare the magnitude of concurrent mother's and infant's behaviors such as visual, tactile, and vocal behaviors, a new method using conditional frequency percentage as an index of the lag sequential analysis on mother-infant interaction was designed. By this method, the lag sequential relation between mother and infant behaviors for both one pair and several pairs could be examined. The subjects were 13 pairs of mother-infant aged 3 months, participating in ‘Tokyo Study Projects’. They were classified into two categories according to the degree of interactions, high activity group (7 pairs) and low activity group (6 pairs). One of the pairs belonging to the low activity group was subjected to the lag sequential analysis of mother-infant interaction at the age of 8, 12 months. The observational data on mother-infant interaction at each month (3, 8, 12 months) were obtained by home visits conducted as a part of ‘Tokyo Study Projects’ in 1977-1979. The main results were as follows. 1) The lag sequential relations on visual and vocal behaviors between mother and infant were different from that of their tactile behaviors. 2) In high activity pairs, following the initiation, the receptor tends to return and continue the same modal behavior. 3) In the low activity pairs, the receptor tends to return but soon stop the behavior. 4) At 3 months, most of the mother-infant interactions were carried out through tactile behaviors. 5) Although, at 3 months, mother's and infant's vocal behaviors didn't occur as many as their visual and tactile behaviors, active interactions between mother and infant followed their mutual vocal behaviors. 6) The amount of mother-infant vocal interactions gradually increased during the first year of life and patterns of the lag sequential relations of these interactions differed also in each period (3, 8,12 months).