Despite the current emphasis in the US on HIV testing and serostatus disclosure as HIV-prevention strategies, little is known about men who have sex with men's (MSM) perceptions of serostatus disclosure by sexual partners. This study used conversation analysis to examine recordings of HIV-test counseling sessions in order to understand how counselors and clients conceptualize and discuss sex partners' disclosure of HIV status. Of 50 test sessions audio-recorded in four publicly funded sites in Northern California, 47 sessions included a discussion about sexual partners' serostatus disclosure, in the vast majority of these (91.5%), counselors and clients avoided directly asserting their knowledge of partners' serostatus. Throughout the discussions, counselors and clients co-constructed the sense of distrust, uncertainty and unknowability of partners' serostatus. The implications of our findings for evaluating the effectiveness of HIV status disclosure as a prevention strategy are discussed.