Inflammasome activation is associated with numerous diseases. However, in vivo detection of the activated inflammasome complex has been limited by a dearth of tools. We have developed transgenic mice that ectopically express the fluorescent adaptor protein, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and characterized the formation of assembled inflammasome complexes (“specks”) in primary cells and tissues. In addition to hematopoietic cells, we have found that a stromal population in the lung tissues formed specks during the early phase of influenza infection, whereas myeloid cells showed speck formation after 2 days. In a peritonitis and group B streptococcus infection model, a higher percentage of neutrophils formed specks at early phases of infection, while dendritic cells formed specks at later time points. Furthermore, speck-forming cells underwent pyroptosis and extensive release of specks to the extracellular milieu in vivo. These data underscore the importance of free specks during inflammatory processes in vivo. Display omitted •The ASC-citrine mouse is a transgenic model that reports inflammasome activation•ASC-citrine retains the function of endogenous ASC•Non-hematopoietic cells are capable of forming ASC specks on activation•The ASC-citrine mouse recapitulates the formation of free specks in vivo In vivo detection of the activated inflammasome complex has been limited by a dearth of tools. Here, Tzeng et al. have developed a reporter mouse model expressing ASC fluorescent protein (ASC-citrine). Mice treated with inflammasome activators have shown increased ASC aggregation, indicating the activation of inflammasome pathways.