Diastereomeric 7-(alpha-ureido-2-amino-4-thiazolylacetyl)cephalosporins are broad-spectrum antimicrobial agents with activity mainly against Gram-negative bacteria. The L-isomers were more potent than the D-isomers. All compounds showed some degree of stability to beta-lactamases. The compounds reached high serum and tissue fluid levels and were excreted unchanged mainly in urine.