1. We evaluated in vitro inhibitory effects of six types of antibiotic, aztreonam (AZT), cefamandole (CMD), cefmetazole (CMZ), cefotiam (CTM), flomoxef (FMOX) and latamoxef (LMOX), on platelet aggregation, using healthy volunteers' blood. Four types--FMOX, LMOX, CTM and CMD--inhibited, in concentration of 2500 micrograms/ml, the secondary aggregation induced by 3.0 microM adenosine diphosphate (ADP), and also inhibited the aggregation induced by 0.5 micrograms/mi collagen. AZT in the same concentration, did not inhibit the aggregation induced by collagen, and it inhibited only ADP-induced aggregation. CMZ, in the same concentration, inhibited neither of the two aggregations. 2. The inhibitory effects of the antibiotics on collagen-induced aggregation were dependent on the concentration of respective antibiotics. When classified by the strength of inhibitory action, LMOX and FMOX were strong, followed by CTM and CMD. The action of AZT and CMZ was weak. In particular, LMOX showed a 32% inhibitory effect at concentration of 50 micrograms/ml, a level near the blood concentration obtained with clinical usual dose. 3. No relationship was observed between inhibitory effects of antibiotics on ADP- or collagen-induced aggregation and the presence or absence of carboxyl group and/or N-methyltetrazolethiol group in the chemical structure.