Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Freshly isolated IEC and HT29 colonic adenocarcinoma cell lines were cultured in the presence or absence of TNFa (33) for 48 h, and culture supernatants were assayed for IL-8 and GROa production by ELISA as described previously (34).
Category: