In an earlier publication, it was shown that consumption of tapwater during the first trimester of pregnancy was associated with a risk of spontaneous abortion among women who attended three prenatal clinics located in northern California between September 1981 and June 1982. Evidence for biased recall was presented. This report demonstrates that the association is limited to women interviewed by telephone and is not observed among those who completed a mail questionnaire. The possibility that the association among women interviewed by telephone is causal and that the lack of association among mail respondents is due to bias, confounding, or apparently chance fluctuations is shown to be unlikely. For instance, the probability is low that, among mail respondents, cases underreported exposure as compared with controls or that random errors were common enough to camouflage a true effect. Demographic differences between mail and telephone respondents also do not explain the results. Interviewers were, however, aware of the outcome of the pregnancy, suggesting that differential reporting of exposure may have been stimulated in the nonblind telephone interview. The pattern of heterogeneity between mail and telephone respondents was also observed for reportable anomalies and tapwater consumption. The data on heterogeneity support the previously published conclusion that differential reporting (biased recall) of this common exposure, tapwater consumption, is the likely explanation for its observed association with the risk of spontaneous abortion. That this association is limited to telephone respondents elucidates the mechanism that apparently produced the bias: that is, the medium of nonblind telephone interviews.