The way that mothers' input assists children with the use of their linguistic resources for negotiating peer interaction was studied. Two two-year-old girls and their mothers were videotaped once a month for eight months while the foursome met at a weekly playgroup. The mothers' language was analyzed in terms of the following questions: (1) Does the input to the children relate to their potential or actual involvement in a joint activity? (2) Do they suggest that one child invite or join the other? (3) What do they say during the course of an established joint activity that threatens to fall apart? (4) When the children do not pick up on the first cues from the input, what modifications do the mothers make in reiterating their suggestions? (5) Given that the mothers intervene to support the joint activity, do they withdraw and steer the peers to each other, and if so, how? The mothers were found to make explicit the means for and advantages of soliciting and acknowledging cooperation. This was particularly obvious in the breakdown of the input as a function of child familiarity with the activity. The mothers stressed establishing shared frames to permit mutual comprehension. It also was found that mothers' input can serve to teach their children how to respect the more open-ended discourse demands of joint peer activity, when to ask, when to tell, and how to converse about what they were doing. Therefore, the input can be seen as a socialization vehicle for helping the child learn to use her existing formal resources to mediate social interaction. (SW)