 Inclusion Criteria: Cohort A: - Pathologic diagnosis of colorectal carcinoma fine needle aspiration or tissue biopsy - Metastatic colorectal carcinoma - Must have previously failed therapy with a fluoropyrimidine and either irinotecan or oxaliplatin; may have received both irinotecan and oxaliplatin; chemotherapeutic agents may have been given concurrently or sequentially. - May have received prior radiotherapy target lesions should not have been irradiated - Bidimensionally measurable disease - Paraffin-embedded tumor tissue available for immunohistochemistry IHC studies of epidermal growth factor receptor EGFr expression archived tissue is acceptable - Tumor-expressing EGFr by immunohistochemistry staining must be 2 or 3 in greater than or equal to 10 of evaluated tumor cells; staining and evaluation to be conducted at a central laboratory - ECOG score of 0 or 1 - Adequate hematologic, renal, and hepatic function Cohort B: - The same inclusion criteria from Cohort A will apply to Cohort B, with the exception of the following tumor expressing EGFr criterion: 