Twenty-two algal cultures grown photoautotrophically were incubated with 4-androstene-3,17-dione and 17β-hydroxy-4-androstene-3-one. Most cultures were capable of effecting transformation. The metabolites were isolated and detected by tic and hplc, and were identified by comparison of chromatographic and/or spectral properties with those of authentic compounds. Most cultures showed the presence of 17-oxidoreductase, some were capable of hydroxylating 4-androstenedione at the 6β- and l4α-positions, and a few were capable of utilizing substrates as an energy and/or a carbon source. Incubations with testosterone showed very little metabolism.