Coccidia were isolated from 122 Belgian broiler farms without clinical coccidiosis. Shuttle programs including robenidin or nicarbazine in the starter (7-14 days) followed by an ionophore or diclazuril in the grower ration were most commonly used. Out of 215 coccidiosis-positive groups, 146 Eimeria acervulina, 65 E. maxima, and 88 E. tenella isolates were tested without further laboratory propagation in 17 sensitivity profiles. For each profile, oocytes were pooled from 9 +/- 4 farms (mean +/- SD) that used the same anticoccidial program and that belonged to the same integrated broiler operation. Each suspension contained an equal number of isolates and oocyst numbers from each farm tested. Each profile included an unmedicated uninfected group, an unmedicated infected group, and 11 medicated infected groups, consisting each of three replicates of three Ross chicks. Medication started at 8 days of age, and each inoculated bird received 50,000 sporulated oocysts at 10 days. Results were related to the anticoccidial program that had been in use. Chemical drugs showed the highest activity against Eimeria, whereas ionophores were less efficacious. Of the latter, monensin (110 ppm) was least active; narasin (70 ppm), salinomycin (60 ppm), and maduramicin (5 ppm) took an intermediate position, and lasalocid (90 ppm) was most active. A 50% improvement in weight gain was obtained in 7 to 10 out of 17 profiles with 100 + 8.35 ppm clopidol/methylbenzoquate (10), 125 ppm nicarbazin (9), 3 ppm halofuginone (8), and 1 ppm diclazuril (7). A 50% improvement in feed conversion was obtained in 7 to 11 profiles with nicarbazin (11), halofuginone (10), diclazuril (9), 33 ppm robenidine (9), clopidol/methylbenzoquate (7), and lasalocid (7). Based on relative oocyst output, the highest activity against E. acervulina was obtained with clopidol/methylbenzoquate (8/16); the highest activity against E. maxima was obtained with lasalocid (6/6), diclazuril (5/6), and halofuginone (5/6); and the highest activity against E. tenella was obtained with diclazuril (8/8), amprolium/ethopabate (5/8), halofuginone (4/8), maduramicin (4/8), and nicarbazin (4/8).