Ceftriaxone is eliminated mainly as unchanged drug; approximately 60% of the dose being excreted in the urine (almost exclusively by glomerular filtration) and the remainder via the biliary and intestinal tracts. The elimination of ceftriaxone is not altered by probenecid
Rocephin® (ceftriaxone) 250mg, 1g and 2g vials Summary of Product Characteristics, Roche Products Limited, updated May 2011

Thirty-three percent to 67% of a ceftriaxone dose was excreted in the urine as unchanged drug and the remainder was secreted in the bile and ultimately found in the feces as microbiologically inactive compounds. The elimination of ceftriaxone is not altered by co-administration with probenecid.
Rocephin® (ceftriaxone sodium) for injection, US Prescribing Information, Genentech USA, Inc, updated November 2010