This study examined whether Blacks who wrote about their experiences with racial discrimination in a laboratory-based disclosure intervention would show greater levels of antibody production in response to an influenza vaccine compared with Blacks who wrote about a neutral topic. Forty-seven participants were randomized to write about their thoughts and feelings around their experiences with racism, or to write about their schedule for the week. Participants wrote on the same topic during each of three 20-min sessions. Blood was drawn prior to the intervention and at 1 and 3 months postvaccination to assess antibody production. Participants in the racism disclosure group produced significantly less antibodies to 2 of 3 viral strains. Post hoc analysis suggests that participants who were unsure about whether their events were due to racism or due to other factors had reduced levels of antibody to 1 viral strain. The attributional ambiguity sometimes associated with racism may inhibit the benefits of disclosure interventions for these types of stressors.