AMESFIT - A MICROCOMPUTER PROGRAM FOR FITTING LINEAR-EXPONENTIAL DOSE-RESPONSE MODELS IN THE AMES SALMONELLA ASSAY

The Ames Salmonella/microsome assay remains the most widely used micro bial test for genotoxicity. In this article, we describe a microcomput er program developed to fit a linear-exponential dose-response model t o Ames assay data for established mutagens. The model includes a linea r term to describe the mutagenic effects of the test agent at low to m oderate doses and an exponential attenuation factor to accommodate dow nturns at high doses due to cytotoxicity. Quasi-likelihood methods are used to obtain estimates of the unknown model parameters, thereby avo iding the need to fully specify the distribution of the experimental d ata. This method of estimation also allows for extra-Poisson variation that is characteristic of counts of mutant colonies of bacteria obser ved in the Ames assay. The particular linear-exponential model used he re was developed for use in the analysis of a recent large-scale colla borative trial using the Ames assay sponsored by the International Pro gramme on Chemical Safety. The use of our program is illustrated using sample data sets taken from that collaborative study. (C) 1993 Wiley- Liss, Inc.