1. In this study mast cells were found to comprise 2.1% of total cells recovered by enzymatic digestion of human lung tissue. 2. This mast cell population consisted of 79% formalin-sensitive, Alcian Blue-positive mast cells and 21% formalin-insensitive, Alcian Blue-positive mast cells. 3. By the use of centrifugal elutriation and subsequent Percoll gradient centrifugation, separate mixed cell populations could be obtained in which the mast cell constituents were either of the formalin-sensitive or -insensitive type. 4. Cell suspensions in which formalin-sensitive cells comprised 97% of mast cells contained approximately 1.34 pg of histamine per mast cell, whereas in preparations in which mast cells were 84% formalin-resistant the histamine content was approximately 4.17 pg of histamine per mast cell. 5. The histamine release upon anti-immunoglobulin E challenge of formalin-sensitive mast cells was greater than the release by formalin-insensitive mast cells. 6. After challenge with opsonized zymosan, only formalin-sensitive mast cells were able to release histamine. 7. Leukotriene C4 release was observed when formalin-sensitive mast cells were challenged with anti-immunoglobulin E. Formalin-insensitive mast cells showed no release of leukotriene C4. 8. Prostaglandin D2 release was observed when formalin-insensitive mast cells were challenged with anti-immunoglobulin E. Formalin-sensitive mast cells showed no release of prostaglandin D2.