The presence of drug residues in ewe's milk samples can be determined by microbial assays. The main limitation of these tests is the large number of false-positive results associated with them. False-positive results can be explained by the interaction of certain substances naturally existing in ewe's milk with the growth of the microorganism used in the test. In this study, milk chemical composition (fat, protein, lactose, total solids), somatic cell counts (SCCs), free fatty acid concentrations, and lactoperoxidase system components were determined in order to investigate their influence on the rate of false-positive results for the BRT and Delvotest microbiological inhibitor tests. Milk samples were obtained after morning milking of Manchega ewes at 15, 30, 45, 60, 75, 90, 105, 120, and 135 days after parturition. The animals did not receive any kind of treatment or medicated feed throughout the experiment. The false-positive rates for BRT and Delvotest were 3.75 and 2.4%, respectively. When the logistic regression model was applied, the percentages of total solids for positive samples were significantly different from those for negative samples (16.90 versus 18.42% for BRT, 16.05 versus 18.45% for Delvotest), while the SCC logarithmic transformation was significantly higher for the positive samples than for the negative samples (5.38 versus 5.11 log units for BRT, 5.32 versus 5.11 log units for Delvotest). Moreover, Delvotest-positive samples exhibited thiocyanate concentrations higher than those of Delvotest-negative samples (8.18 mg/liter versus 6.85 mg/liter). Further analyses are needed to confirm the possible presence of antimicrobial residues in this particular type of milk sample.