Serum immunoglobulin (IgG, IgM, IgA) and complement (C3, C4) levels were measured by radial immunodiffusion technique in a group of 16 Nigerian patients with tropical pyomyositis (TP). Sixteen healthy Nigerians, age- and sex-matched with the patients, were also included in the study as control subjects (CS). A significantly low level of circulating IgM (P < 0.01) and elevation of the serum levels of IgG (P < 0.01) and IgA (P < 0.01) were observed in TP compared to CS. Mean serum complement (C3, C4) levels were found to be similar in patients and controls. Microbiological culture tests showed that 14 pus specimens (88%) and 3 blood specimens (19%) from the patients were positive for Staphylococcus aureus. It was therefore proposed that IgM antibody deficiency (primary or acquired) against S. aureus may be the cause of tropical pyomyositis in Nigerians.