The speech of depressed and well mothers during play with their infants at two months was compared on dimensions of structure and semantics. No differences between maternal groups were found on measures of complexity and syntax. However, the speech of depressed women expressed more negative affect, was less focused on infant experience, and tended to show less acknowledgement of infant agency. Speech style of depressed women also varied according to infant gender. Regression analyses indicated that the quality of maternal communication with the infant, and particularly the focus of speech, mediated the association between depression and infant cognitive development in the first 18 months.