Frequently it is necessary to search for oocysts of Cryptosporidium sp. in the stools potentially contaminated with human immunodeficiency virus (HIV). For the protection of the laboratory personal handling fecal material formalin has been considered useful. However, this disinfectant has not been evaluated when the stool examinations were performed with the technique of flotation-centrifugation in concentrated solution of sugar and with that of "Coprotest". Our experience indicates that the use of formalin may interfere with the results when the first of these methods is used, but it does not alter the results of the second one. This information is of importance for the future assistential and scientific studies.