Oligosaccharide fragments were prepared by partial acid hydrolysis of sodium alginate and consisted of oligomannuronate (OM) and oligoguluronate (OG) blocks. Effects of the OM and OG blocks on penicillin G production by P. chrysogenum were investigated. The oligosaccharides were found to cause significant increases in penicillin G yields. OM blocks at concentrations 10 to 100 mu g/mL were used to further evaluate the effects of the oligosaccharides, and were found to enhance the production of penicillin G in shaken flask cultures of P. chrysogenum P2 (high penicillin producer) and NRRL 1951 (low penicillin producer) at the test concentrations. There was an approximately 50% maximum increase in penicillin G yield from biomass in P. chrysogenum P2 cultures and 150% in P. chrysogenum NRRL 1951 cultures, when compared to control cultures without the oligosaccharides.