Pharmaceutical compounds are identified in environmental matrices in the order of magnitude of ng L-1 to ??g L-1. Among the drugs, antibiotics have been receiving special attention due to the problems caused to aquatic biota. The aim of this study was to evaluate the cytotoxicity of Amoxicillin and Potassium Clavulanate, in isolated and associated forms, to marine mussels Perna perna through neutral red retention time assay (NRRT), which assesses the stability of lisossomal membrane of test-organisms hemocytes. Amoxicillin caused cytotoxicity to the mussels in concentrations of OEC= 1 ng L-1, IC25-24h= 0.44 ng L-1, IC25-48h= 1.19 ng L-1 and IC25-72h= 0.85 ng L-1, Potassium Clavulanate was cytotoxic at concentrations of OEC= 10 ngL-1 in 24h; 50 ng L-1 and 100 ng L-1 at 48h and 72h. The inhibitory concentration values (IC25) were: 3.11 ng L-1, 3.45 ng L-1 and 3.43 ng L-1 at 24h, 48h and 72h respectively. In the test conducted with the combination of drugs, all concentrations were cytotoxic to mussels in 48h. In 72h only 40 ng L-1 Amoxicillin + 10 ng L-1 Potassium Clavulanate and 200 ng L-1 Amoxicillin + 50 ng L-1 Potassium Clavulanate were cytotoxic. The IC25 for Amoxicillin was 1.67 ng L-1 in 48h and 1.36 ng L-1 in 72h. For Potassium Clavulanate was 0.42 ng L-1 in 48h and in 72h was 0.34 ng L-1.