RBE determinations of special relevance to the quality factor assigned for radiation protection purposes are those relating to the effects of special importance at low doses, namely carcinogenesis and mutagenesis. Measurements of RBE that enable the maximum value of RBE, namely RBEM, to be determined at low doses require data points as low as 0.1 Gy or even 0.01 Gy of high LET radiation. Corresponding data points as low as 0.5 Gy to 0.25 Gy or less of low LET radiation are also needed. Relatively few such measurements have been made, but many more are available now than formerly. A review of recent RBEs for tumour induction, life shortening, transformation, cytogenetics and genetic endpoints, which updated an earlier review, indicates a broad range of results. The principal findings are that X rays are more effective than hard ? rays at low doses by a factor of about 2, and that fission neutrons, alpha particles and heavy ions may be 30-50 times more effective, on the average, (some endpoints give higher, some lower values) than hard ? rays. The data would seem to indicate that in order to provide approximately equal protection against the risks at low doses from all radiations, adjustments upward in the quality factors for high LET radiations need to be considered.