The amount of microbial material found in various grades of 1980 samples of American cotton was investigated. The contents of endotoxin, gram negative bacteria (GNB), gram positive bacteria (GPB), and fungi were determined for 296 classers' raw cottons. Serial 10 fold dilutions were made and 0.1 milliliter of each dilution plated out on tripticase soy agar with cycloheximide for GPB and GNB determinations or on sabouraud agar with chloramphenicol for fungi determinations. Endotoxin determinations were made using a modified Limulus amebocyte lysate method. Relations among microbiological and fiber parameters were sought. Microbial concentrations varied enormously between samples. GPB counts ranged from 2 to 53000 colony forming units per milligram (cfu/mg); GNB counts ranged from 5 to 230000cfu/mg. Numbers of fungi ranged from collection of cotton varied from 0.01 to 83 nanograms/mg. For the collection as a whole, GPB values and fungi concentrations showed no consistent relationship to grade division or color group as assigned by the classer. GNB counts and endotoxin concentrations were related to visually assigned classers' color groups. For the whole collection, fiber length and length uniformity, measures of fiber maturity and fineness were inversely and significantly correlated with endotoxin values. Trash grade, gray and yellow hue values, and GPB and fungal counts all positively and significantly correlated with endotoxin values. Cottons from the Southwest and Southeast regions had significantly higher endotoxin and GNB values than those from other regions. The authors conclude that significant differences in microbiological concentrations occur between these crop cottons from several United States growing regions. These may affect the adverse health effects caused by cotton dust.