The relation between infant-sibling affective involvement and the attachment security of each child to the mother was examined in the present laboratory investigation. In mothers' presence, securely attached infants were less likely to protest and aggress against mothers and older siblings when mothers played only with the older child. In mothers' absence, more secure older siblings were more likely to respond to infant distress with caregiving than were less secure older siblings. Although infant attachment behavior to older siblings was rare, it occurred only when the older sibling was more secure. Sibling dyads with a secure infant and a more secure older child appeared to be most likely to develop nonantagonistic relationships, whereas sibling dyads with an insecure infant and a less secure older child appeared least likely to do so. These findings were discussed in terms of the putative role of attachment security in shaping sibling bonds.