In applying linguistics to the task of analysing how agentivity is construed through verbal interaction, scholars often equate social agency with grammatical agency, and in particular with the grammar of transitivity. The difficulty I want to address in this paper is that we may miss other important, systematic and contrastive patterning in the agentivity with which social actors and other entities are depicted, because such agentivity is realized through a range of dispersed linguistic resources. Systemic Functional Linguistics can provide a useful framework for co-ordinating the contribution of these resources to the overall construal of agency in a text or set of texts. It does this best when it focusses on bringing out the particular stratal alignments that characterise different contexts. The paper draws on a study of treatment decision-making in HIV medicine as an example of a social context where choices in the construal of agency make a crucial difference to choices of professional and institutional practice. In this study the construal of agency was taken as a chief source of evidence about whether doctors and patients engage in shared decision-making, and it was also seen as a strategy which doctors and patients can use to open up or close down opportunities for shared decision-making. A key finding was that doctors and patients in HIV medicine often construe the agency of one participant as a resource for the agency of another, rather than construing the agency of one participant as competing with the agency of the other. In particular, it is where doctors and patients construe each other as semiotic agents that shared decision- making seems most likely to occur.