Worker exposure to N,N-dimethylacetamide (DMAC) in an acrylic fiber manufacturing facility was measured, over a one year study period, by full-shift (12 hours) personal air monitoring for DMAC and by biological monitoring for levels of DMAC, N-methylacetamide (MMAC), and acetamide in spot urine samples. Ninety-three of 127 male workers in seven job classifications in the solution preparation and spinning departments of the plant were monitored on the second consecutive work day following at least three days off for the first 10 months of the study and on both the first and second days during the study's final two months. Post-shift urinary MMAC levels were significantly correlated (p<0.0001, r-square=0.54) with DMAC in air levels. An air level of 6.7 ppm 12-hour TWA corresponded to a urine MMAC level of 62 mg/g creatinine in a post-shift spot urine sample obtained following the second consecutive work day. To minimize exposure misclassification due to variability in the regression relationship, a level of 35 mg MMAC/g creatinine in a post- shift spot urine sairttple was recommended as a biomonitoring index. Post-shift urine MMAC levels did not appear to plateau at higher air levels, nor did it appear that the DMAC demethylation metabolic mechanisms became saturated at TLV-level air exposure levels. Urine MMAC levels in post-shift samples obtained the second work day appeared to be greater than levels in post-shift first day samples but the number of days until this post-shift level would plateau could not be determined from this study. A level of 35 mg MMAC/g creatinine in a post-shift urine sample obtained after as many consecutive work days as feasible appears to be an acceptable biomonitoring index of exposure to DMAC for workers working a 12-hour work shift.