As part of the 1986 Census of Central Los Angeles County, several experiments were conducted to evaluate the effect of procedures designed to encourage public cooperation. High mail response rates are critical to an efficient and cost-effective national census. While mail response rates have traditionally been high for the decennial census, we cannot afford to assume that the past is a perfect predictor of the future. Therefore, it is critical for the Census Bureau to develop and test new procedures designed to improve and/or maintain high mail response rates. Previous social science research suggests that the use of follow-up reminders can increase response rates in mail surveys (Heberlein and Baumgartner, 1978; Nederhof, 1983). Follow-up reminders were also successful in increasing mail response rates in the 1982 Census of Agriculture (Ruggles, Dea, Kwok and Carmen, 1984). A study to assess the feasibility of using a follow-up reminder in a population census indicated that a reminder postcard increased response rates 8 percentage points in a test census in which baseline response rates (for the different form types used) ranged from 42 to 54 percent (Marquis and Sedlacek, 1985).