Following cannulation of the right external jugular vein and the efferent duct of the right caudal mediastinal lymph node (the caudal end of this node having been ligated to cut off the inflow of systemic lymph), sheep were each given one of four "cephalosporins" (cefazolin, moxalactam, cefoperazone, or ceftriaxone) as single doses injected iv over 30 min. All of the drugs appeared in the pulmonary lymph during iv infusion. Peak concentrations in the lymph were attained at 5 min postinfusion with cefazolin, cefoperazone, and ceftriaxone; the peak for moxalactam was attained at 30 min postinfusion. Cefazolin and cefoperazone penetrated better than did ceftriaxone, which penetrated better than did moxalactam. The concentrations of moxalactam, as compared with the other drugs, declined more gradually in both venous blood and pulmonary lymph. In view of the prompt entry and transit through the lungs and the high concentrations attained in the pulmonary lymph, these drugs should be effective in the treatment of pneumonias caused by susceptible bacteria.