Purpose: Understanding variability sources in early language interaction is critical to identifying children whose development is at risk and designing interventions. Variability across socioeconomic status (SES) groups has been extensively explored. However, SES is a limited individual clinical indicator. For example, it is not generally directly modifiable. The purpose of this study was to examine if child language ability, input quantity and quality, and dyadic interaction were associated with modifiable caregiver characteristics--self-efficacy and developmental knowledge. Method: We conducted secondary analyses using the baseline data (n = 41 dyads enrolled, n = 30 analyzed) from a longitudinal study. Mothers and children (1;0-2;3 [years;months]) in low-income households completed demographic questionnaires, self-efficacy and developmental knowledge measures, child language assessments, and interaction samples. We used linear regression models to examine the relationship between self-efficacy, developmental knowledge, and outcomes. Results: Child receptive and expressive language scores were significantly associated with mothers' self-efficacy, knowledge, and Efficacy × Knowledge interaction. Specifically, maternal self-efficacy was positively associated with child language only in the context of high developmental knowledge. Neither self-efficacy nor developmental knowledge was significantly associated with the number of total or different words mothers produced. However, self-efficacy was significantly and positively associated with the rate of child-initiated conversational turns per minute, controlling for the number of child utterances. Mothers with higher self-efficacy responded more readily to their children than those with lower self-efficacy. Conclusions: Child language ability and interaction quality vary based on modifiable parent characteristics. Modifiable individual characteristics should be considered in early language interaction within and across SES groups.