The author''s adaptation of Beyerinck''3 "auxanographic" technique is useful in the preliminary steps of identifying growth factor requirements of microorganisms. Cells of the organism to be investigated (Aspergillus nidulans in the study described) are distributed in a nutritionally deficient agar which is allowed to set in a petri dish. Possible required nutrients are then spotted on the agar surface, enabling mutants for the corresponding deficiency to grow. It is possible by streaking different substances at right angles to obtain patterns of growth indicating competitive inhibition or a requirement for two growth factors.