The present article reports a study of communicative behaviour among mothers and infants who were grouped according to the ratings of sensitivity and co-operation, respectively. The participants were 27 Finnish-speaking mothers and their 10-month-old first-born infants (13 boys and 14 girls). The study is descriptive by nature, and the data were therefore treated in terms of descriptive parameters. According to the results, maternal sensitivity was associated with general activity in communication as well as the frequency of responses, while infant co-operation was associated with the frequency of intentional communicative acts. As for more detailed aspects of communicative style, for example, the use of different kinds of communicative means, only some slight associations were found between them and the ratings of sensitivity and co-operation. The dyadic nature of maternal sensitivity is emphasized in interpreting the findings. (Contains 6 tables and 3 figures.)