Leachate and soil samples collected from different tillage systems were analyzed for atrazine using gas chromatography (GC) and an enzyme-linked immunosorbent assay (ELISA) based on magnetic particle technology. Solid-phase extraction (SPE) was used to concentrate atrazine residues in leachate samples and soil extracts before GC analysis. Atrazine concentrations determined by GC ranged from 0.1 to 600 micrograms L-1 for water samples and from 1.0 to 700 micrograms kg-1 for soil samples. Atrazine concentrations in 92 leachate samples as determined by ELISA were well-correlated (R = 0.97) with GC levels over the entire concentration range. Soil samples (215) were prepared and analyzed by three combinations of extraction/detection methods: 1) conventional extraction for GC/detection by GC analysis; 2)conventional extraction for GC/detection by ELISA analysis; 3)extraction for ELISA using a commercially available field kit/detection by ELISA analysis. Methanol (MeOH) in water was the common extractant. Although the initial comparison of soil extracts between the two different systems (Method 1 versus Method 3) was favorable (R = 0.97), two-thirds of the samples contained levels below the lower threshold for atrazine detection by both methods and some extracts were perceived to provide unfavorable substrate conditions (> 10% MeOH). Elimination of these data points reduced the correlation value (R = 0.77). To determine possible sources of variability, the extraction and detection methods were examined separately. In a comparison of extraction methods (Method 2 versus Method 3), ELISA analysis of kit extracts underestimated (R = 0.71) atrazine levels compared to those conventionally extracted, suggesting that differences in extraction time between methods may have accounted for reduced kit efficiency. Where detection methods (Method 1 versus Method 2) were compared on specific extracts (< 10% MeOH), good agreement (R = 0.99) was achieved between ELISA and GC values, illustrating that control of extractant concentration is critical in using this assay for atrazine detection in soil.