Summary A range of new anthracyclines, structurally related to adriamycin (ADM), has been synthesised and studied in vitro. Three compounds described in this paper (Ro 31-1215; Ro 31-1741; Ro 31-2035) are all 4-demethoxyanthracyclines. In the mouse mammary tumour cell line, EMT6/Ca/VJAC, using a 1 h drug exposure followed by colony formation as the response endpoint, we found Ro 31-1215 and Ro 31-1741 to be 2-3 x and 4-7 x more potent then ADM, whilst Ro 31-2035 was 3-4 x less potent. For continuous drug exposure and suppression of population growth as the endpoint, the potency of Ro 31-1741 was similar to that of ADM, whereas that of Ro 31-1215 was 1.5-2 x higher and that of Ro al., 1978;Kaye & Merry, 1985). The mechanism of such resistance is currently the subject of much ongoing laboratory work and a variety of strategies for overcoming resistance are being investigated (Tsuruo et al., 1983;Skovsgaard et al., 1984).Over the last 10 years a large number of analogues of ADM have been produced with the major objective of finding a drug which is less cardiotoxic for a given amount of anti-tumour effect (Naff et al., 1982