In the paper, environmental samples collected from sources implicated in an epidemic of Legionnaires' disease were surveyed. Although no cultures were recovered from environmental samples, numerous cells were observed by fluorescent microscopy when anti-L, pneumophila group 1 antibody was used. Similar observations have often been made by others. To study the loss of culturability, L. pneumophila strains were maintained in a microcosm (vessels containing sterilized environmental water) and assayed at intervals for growth on appropriate media, and lethality for chick embryos. At 4 deg C, the decimal rate of decline of colony forming cells was approximately 29 days; at 37 deg C it was 13 days. Results indicate that samples that do not contain culturable cells, may contain cells that are viable, as demonstrated by their pathogenicity for chick embryos. The fluorescent antibody assay may provide a valuable indication of the presence of such viable but non-culturable cells.