In 1983 a bulk sample of cotton dust DB 11-82 was divided into a number of bags all of which were stored in a cold room at 10 C. Acute exposure of guinea pigs during 1987 to the dust samples revealed that several samples had reduced pulmonary toxicity. Evaluation of the microbiological content of the samples indicated variation in number of viable Gram negative bacteria and ratio of Ps. syringae:E. agglomerans. However, residual toxicity could not be correlated with either of these factors. These results indicate that pulmonary toxicity of cotton dust can change with storage and imply that analysis of toxicity must be performed regularly when using stored materials.