In an experiment designed to examine nonresponse bias, either the March of Dimes or the University of Michigan was identified as the sponsor of a survey mailed to individuals whose level of support for the March of Dimes was known. The response rate was higher to the university survey, but support for the March of Dimes increased survey participation to the same extent in both conditions. As a result of the overrepresentation of supporters of the organization, both surveys showed nonresponse bias for variables linked to support. The bias was greater, however, when the sponsor was identified as the March of Dimes. Thus, the university sponsor brought in not only more of the sample but also a more representative sample on variables related to support for the March of Dimes. Overall, the magnitude of the relationship between support for the organization and nonresponse was not a strong predictor of the magnitude of the nonresponse bias. The results demonstrate that the simple “common cause” model of nonresponse will not always apply, and that the model should be extended to incorporate multiple auxiliary variables.