The in vitro susceptibility of gentamicin fractions against Fusarium growth was the subject of this retrospective study. Fusariosis was earlier an exceptionally rare human disease and an unrealistic idea to treat soil saprophytes and plant pathogens with expensive antibiotics such as gentamicins or their minor components. Disseminated fusariosis is now the second most frequent lethal fungal infection after aspergillosis especially in neutropenic patients with hematologic malignancy. Results of this study obtained between May and November 1973 were interesting but not practicable and remained unpublished. Seven Fusarium and 28 other fungal strains were tested for their susceptibility to gentamicin B1. The anti-Fusarium activity of gentamicin B1 was between 0.2 and 3.1 mu g/ml minimum inhibitory concentration (MIC) values. The MIC values of clotrimazol and amphotericin B against Fusarium species were significantly higher, 3.1-12.5 mu g/ml and 3.1-50 mu g/ml, respectively. Gentamicin B1 and its structurally related congeners including hygromycin B, paromomycin, tobramycin (nebramycin factor 5), nebramycin (nebramycin factor 4), and sisomicin exerted strong in vitro inhibition against Fusarium species between 0.2 and 12.5 mu g/ml concentrations. The antibacterial MIC concentration of gentamicin B1 tested on 20 bacterial strains ranged between 0.1 and 50 mu g/ml. Gentamicin B1, a minor fraction of the gentamicin complex, inhibited effectively the growth of Gram-positive (Staphylococcus, Streptococcus, Bacillus subtilis) bacteria and Gram-negative (Escherichia coli, Salmonella, Proteus, Pseudomonas) pathogens. Gentamicins and related aminoglycoside antibiotics are used in medical practice. It is proposed that due to the increasing incidence of fusariosis and drug resistance, gentamicin components, particularly minor fraction B1 and related aminoglycoside antibiotics, could be tested for their in vivo activity against fusariosis and aspergillosis either alone or in combination with other antifungal agents.