The nature and lateral variation of a host rock limestone in the Tri-State zinc-lead district were petrographically studied within a single horizon, the M bed. This bed is a massive, cherty, crinoidal limestone in the Tri-State district of southwestern Missouri, northeastern Oklahoma and southeastern Kansas and is a part of the Mississippian Keokuk Formation.One hundred and twenty specimens from this bed were collected under-ground from six pull drifts in five mines: West Side, Grace B, Big John, Little Greenback, and Mudd Mines. The specimens were collected at regular intervals, 10 to 50 feet, beginning at the edge of an ore body and progressing away from the zone of mineralization and ore. Ninety-four thin sections prepared from the limestone specimens were described and classified according to Folk's classification of carbonate rocks. According to this classification M bed limestone is predominantly a sparry allochem.Distally from ore bodies the M bed limestone consists predominantly of allochems, mainly crinoids, and microcrystalline calcite, sparry calcite and microspar are much less abundant constituents. Toward ore bodies, the constituents of the M bed limestone are altered by the solutions that emplaced the ores. The allochem content progressively decreases, whereas the sparry calcite content increases. This gradual change in the limestone constituents toward the margin of ore bodies generally begins at a distance of 40 to 80 feet from the ore bodies. Microcrystalline calcite and microspar exhibit no significant lateral variations.