Mecillinam, a beta-amidinopenicillanic acid derivative, was combined with ampicillin, amoxicillin, carbenicillin, cephalothin, cefamandole, and cefoxitin and tested against most members of the Enterobacteriaceae and Pseudomonas. Synergy was demonstrated with selected isolates of most of the organisms tested. Isolates highly susceptible to mecillinam (minimum inhibitory concentration, <0.8 mug/ml) were not synergistically inhibited by addition of another beta-lactam antibiotic. Synergy of mecillinam and a beta-lactamase-resistant penicillin, cloxacillin, was demonstrated. In media of osmolality >10 mOsm or of conductivity >6 mS, mecillinam and beta-lactam antibiotics showed synergy in most instances, whereas at low osmolality and conductivity the activity of mecillinam is so great that synergy cannot be demonstrated. The proportion of mecillinam to beta-lactam antibiotic that will be synergistic ranged from 100:1 to 1:1 to 1:100. Mecillinam did not increase the activity, minimum inhibitory concentration or minimum bactericidal concentration values, of beta-lactam compounds against streptococci, staphylococci, clostridia, listeria, or bacteroides. Synergy was not demonstrated with combinations of mecillinam and aminoglycosides (kanamycin, gentamicin, tobramycin, amikacin), chloramphenicol, tetracycline, or polymyxins.