This study investigates the interactional function of Japanese interactive markers yo and sa in conjunction with the notion of involvement, and sheds light on some issues of verbal exchange in spoken discourse. The study argues that these markers commonly share the function of signalling the speaker’s attitude of inviting the hearer’s involvement in a ‘monopolistic’ manner, by which the speaker is committed to enhance his/her position as a deliverer of the contents/feeling conveyed in the utterance. Further, each marker indicates the speaker’s monopolistic attitude in its unique way: yo signals the speaker’s monopolistic attitude of ensuring the hearer’s understanding of the contents/feeling conveyed in the utterance, while sa signals the speaker’s monopolistic attitude of presenting the contents/feeling conveyed in the utterance as a matter of course for him/her. Based on these functions, their co-occurrence restrictions with modal expressions and their use in social contexts are also discussed.