In vitro susceptibility of Actinobacillus actinomycetemcomitans (A.a.) serotypes to selected antimicrobial agents was investigated by the agar dilution method on supplemented Mueller-Hinton test medium. Eighty-three A.a. strains, 80 recent isolates from 40 periodontally healthy or diseased subjects, and three type strains were included in the study. Serotype a represented 20, serotype b 32, serotype c 17, and serotype e 7 and nontypable 4 of the tested strains. The most effective drugs against all A.a. serotypes in vitro were cefaclor, cefuroxime, tetracycline hydrochloride, doxycycline, trimethoprim-sulfamethoxazole (cotrimoxazole), and ciprofloxacin, which inhibited 100% of the strains at 4.0 micrograms/ml, 4.0 micrograms/ml, 1.0 microgram/ml, 2.0 micrograms/ml, 0.06 microgram/ml, and 0.015 microgram/ml, respectively. Serotypes a and e were more susceptible to cefaclor and cefuroxime than were serotypes b and c; 100% of the first two groups were inhibited at 2.0 micrograms/ml and 1.0 microgram/ml. Ampicillin inhibited 92% of the tested strains at 1.0 microgram/ml. Serotype b was always susceptible to ampicillin. Metronidazole exhibited the best activity against serotype a strains. The lowest minimal inhibitory concentration values for benzylpenicillin, ampicillin, erythromycin, doxycycline, and metronidazole were encountered among serotype b isolates. The results of the present study indicate minor differences in the in vitro antimicrobial susceptibility patterns of different A.a. serotypes, except to metronidazole. Also, the new oral cephalosporins and cotrimoxazole, rare antimicrobial agents in periodontology, showed promising efficacy against all A.a. strains.