After hydrolysis of chitin in 6 M HCI, the glucosamine produced was assayed calorimetrically. The pH of the hydrolysate was adjusted to a value close to three by addition of Na acetate; this procedure avoids the elimination of excess acid by evaporation under reduced pressure or freeze-drying. Under these conditions, the amount of glucosamine determined by the assay represented an average of 90% of the amount which would result from a total hydrolysis of the chitin. The method was used to assay the chitin in the mycelia of basidomycetes obtained in vitro. The measured amount of glucosamine was proportional to the mycelial biomass and allowed the estimation of fungal growth.