Multiple-choice standardized achievement tests of English vocabulary and reading comprehension and of mathematics were administered to samples of 592 grade eight students and 615 grade five students. Two forms of each test unit were prepared. The control groups took forms containing items with four responses, while the experimental groups took forms which had an additional response of I don't know. A few fictitious vocabulary items having no right answers were included in each of the English test units. In grade eight tests the mean scores of the control groups were higher. For the grade five samples there were no differences in mathematics; and differences in English (the control group obtaining higher scores) were found only for low ability students. Item discrimination indices obtained from the two forms did not show any significant differences. There was a negative linear relationship between percentage choosing the I don't know response and percentage correct. In general, lower ability students used the response more often than those with higher ability excePt in the case of the fictitious items. Sex differences were also found to be a factor in the use of the I don't know response. (Author/MV)