Lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativus L.), and marigold (Tagetes patula L.) transplants were grown from seed in leached and unleached media containing 0%, 25%, or 50% (by volume) peat and/or fresh or aged spent mushroom compost with 50% vermiculite. Reduced growth and symptoms of ammonium toxicity were seen in transplants grown in fresh spent mushroom compost. Transplants grown in 0% or 25% compost were larger than those grown in 50%, probably due to high salinity in 50% compost. Leaching reduced media soluble salts and generally improved plant yields. K and Ca were higher and P and Mg were lower in the tissue of transplants grown in spent mushroom compost than of those in the peat-lite control mix. High quality transplants were produced in 25%, aged spent mushroom compost, while acceptable plants of slightly reduced quality were produced in 50%, aged compost.