The usual method of estimating the mutual interference constant, m, assumes a linear (type I) functional response of predators. In the cases where the response is not linear, the application of the method introduces a bias in the estimation of the searching efficiencies. It is shown that, as a consequence, the value of m is underestimated. A new method is proposed, which allows for a type II functional response due to a handling time. A comparative analysis of 15 data sets from the literature shows that the proposed method gives values of m that are consistently higher than those estimated by the traditional method. The new method calculates the parameters with nonlinear regression and provides standard errors for the estimates. Therefore, the reliability of the searching efficiencies, the handling time and the constant m can be quantified. Very few of the interference constants are significantly different from m=1. This special value implies that the functional response is a function of the ratio of prey and predator densities. These empirical findings support the suggestion of Arditi and Ginzburg (1989) that the functional response might often be ratio-dependent, especially in complex and heterogeneous situations.