The Antimicrobial activity of aspoxicillin (ASPC) in terms of minimum inhibitory concentration (MICs) was compared with those of other penicillin antibiotics (PCs) against clinical isolates sent to us from medical institutions throughout Japan in 1988, 1990 and 1992 and strains isolated and identified from samples collected from patients with various infections. 1. The MIC80's of ASPC against Staphylococcus aureus, Enterococcus spp., Escherichia coli, Bacteroides fragilis group were almost the same as those against these isolates in 1985 to 1986. 2. A trend for increasing susceptibility to PCs including ASPC was observed in the isolates of S. aureus and Haemophilus influenzae. This trend in S. aureus was attributed to the appearance of non beta-lactamase producing strains associated with the development of highly resistant strains among the methicillin-resistant S. aureus (MRSA) as well as to a tendency toward yearly decreasing frequency of MRSA. The trend for the increased susceptibility in H. influenzae was related to the decrease in the number of beta-lactamase production strains. 3. The frequency of the strains highly resistant to PCs including ASPC increased. 4. No PCs-resistant strains were observed among the so-called beta-streptococci, while among alpha-streptococci and Streptococcus pneumoniae there was a trend for decreasing frequency of strains with lower susceptibility to PCs or those with resistant to PCs. These strains may be variants which were also resistant to cephems and had penicillin-binding proteins (PBPs). Meanwhile, a high frequency of highly PCs-resistant strains were noted among Enterococcus faecium. In view of the fact that the PCs-resistance of E. faecium is known to be related to PBPs, the pattern of the susceptibility of the recent clinical isolates to beta-lactams is considered to be multimodal.