Ewe milk samples from different ovine dairy farms from the Castilla-La Mancha region of Spain were collected from bulk tanks to estimate the occurrence of antibiotic residues in raw and heated (82 degrees C, 10 min) milk by the Delvotest SP test. The month of collection, somatic cell counts, and bacteriology were analyzed and examined by means of a logistic regression model. The screening of a total of 2686 raw milk samples showed 1.7% "positive" and 2.1% "doubtful" results, which decreased after heating treatment to 1.3% and 0.4%, respectively. "Positive" and "doubtful" samples were identified by penicillinase and p-aminobenzoic acid solutions, and the majority of them corresponded to antimicrobials different than beta-lactams or sulfonamides. By applying a logistic regression model, a significant effect of month of collection and bacteriology was observed in the initial screening and after the heat treatment. The highest percentages of "positive plus doubtful" results were observed in late summer-early autumn. A slight peak was also observed in spring in raw milk samples. Bacteriology was positively correlated with "positive plus doubtful" results.