The literature indicates that the effects of sources of demographic biases on fairness in classroom assessment (CA) are under-researched in the Iranian higher education context. Hence, this study aims to explore the Iranian university teachers' perceptions of the effects of demographic biases (e.g., gender, ethnicity, and socioeconomic (SES)) on their assessment fairness. With this aim, fifteen university teachers were selected using a criterion sampling method at Ayatollah Borujerdi University, Iran. To gather the required data, a reflective written statement was completed by the participants. The participants' responses were translated verbatim into English and subjected to a standard thematic coding analysis. Findings yielded three recurring themes: 'gender bias is prevalent in assessment practices', "ethnicity bias affects adversely assessment practices," and 'SES bias jeopardizes fairness in assessment practices'. The results evidenced that gender, ethnicity, and SES biases can act as a major source of score pollution in CA. This study ends with proposing a range of implications for different testing stakeholders.