Detailed serological investigations were carried out in forty-nine active kala-azar (KA) cases in North Bihar, India. Various classes of immunoglobulin (IgG, IgA, and IgM) and third component of complement (C3) levels were determined in these sera and results were compared with those obtained in normal controls. Antibody titres were determined by the indirect haemagglutination (IHA) method using soluble Leishmania antigen. Immunoglobulin G and M class-specific antibody titres were also determined separately by the enzyme-linked immunosorbent assay (ELISA) method. Polyclonal hypergammaglobulinaemia with marked increase in serum IgG (and to a lesser extent in IgM) level was a rather common feature in the majority of these sera. Much of this immunoglobulin increase, however, appeared to be non-specific in nature and no absolute correlation could be noted between serum IgG or IgM levels and corresponding IgG or IgM antibody titres. Significant decrease in serum C3 level was observed in KA and this decrease was found to be independent of immunoglobulin levels or specific antibody titres. A fairly good correlation between aldehyde test and serum IgG level was evident from this study. Aldehyde-positive KA sera usually gave higher antibody titres than aldehyde-negative ones. Anti-leishmanial antibodies belonged mostly to IgG class although some IgM antibodies were also demonstrable. The latter class of antibodies probably appeared early in KA infection although their serological specificity was poorer to IgG antibodies. Out of forty-nine KA sera examined in this study thirty-six (73.5%) gave positive IHA titres while forty-six (93.9%) were positive by IgG-ELISA which appeared to be a highly specific and sensitive serodiagnostic method particularly for the early detection of KA cases.