In the first part of this paper we report on a new method we have developed for the study of the genesis and regulations of the behavior of infants in interactive situations. Detailed information is given on the following aspects: 1. The technical characteristics of the device that allows the infants to be seated in an upright position; 2. The different periods of the experimental procedure; 3. The method used for analyzing the behavioral data. As infants were video-taped without any interruption (real-time record), the behavioral data were analysed in two different ways: A. Macroscopic analyses were carried out using 84 items and 12 categories of behavior. These analyses allowed us to measure the temporal evolution of a limited set of easily recognized behaviors from one experimental period to the next and one experimental session to the next at one-week time intervals; B. Microscopic analyses were carried out with a special software program (THEME) using 222 items divided among 14 categories of behavior. This program was created by Magnusson. It allows the worker to search for significant patterns in sequential events which are recorded in real time, i.e. intra-individual behavioral structure as well as interindividual relationship (interaction). Information is given on the theoretical model and the pattern identification method. The first data can be thus summarized: 1. Four month old infants show a high level competence in their visual attention span in relation to each other; 2. They appear to have varied and complex behavior and interaction processes; 3. Each child has his or her own behavior dynamics throughout each experimental session and from one period to the next; 4. There are some special relationships between infant-infant interactions and the modalities of interaction between each infant and its mother; 5. Periodical behavioral "outbursts" are observed, which might in themselves constitute organizers of behavior; 6. It also makes clear the existence and structure of significant intra- and inter-individual behavioral patterns some of which are particularly complex (patterns are defined as significant connections between two items at least, within a critical time interval, calculated by THEME); 7. It reveals the function of these patterns in mutual adjustment processes between both infants throughout the experimental session.