Alternatives to M-estimation for robust estimation of the median lethal dose in biological assays are developed. A class of link functions based on the Student-t distribution is proposed, where degrees of freedom are estimated from the data by maximum likelihood. Other alternatives include slash and finite mixture distributions. For bioassays from a pharmaceutical company, these methods extend the standard probit and logistic models, as well as the Huber's M-estimator. They are also applied to several standard examples from the literature.