Clarithromycin acts by inhibiting microsomal protein synthesis in susceptible organisms mainly by binding to the donor site on the 50S subunit of the bacterial ribosome and preventing translocation to that site. Clarithromycin is active against most Gram-positive bacteria and Chlamydia, some Gram-negative bacteria and Mycoplasmas. Clarithromycin’s activity is the same as, or greater than, that of Erythromycin in vitro against most Gram-positive bacteria. Clarithromycin is more acid-stable than Erythromycin and therefore, is better tolerated. Clarithromycin has twice the activity of Erythromycin against H. influenzae. Most species of Gram-negative bacteria are resistant to Clarithromycin because of failure to penetrate the target.