Monoclonal antibodies (Mab) were raised against haemoglobin (Hb) associated with Plasmodium falciparum protein and used to develop an ELISA, measuring circulating levels of released Hb. This assay was evaluated in different malaria patients in parallel with ELISA assays for C-reactive protein (CRP) and haptoglobin. Levels of Hb were negatively associated with levels of haptoglobin. Increased levels of serum Hb and CRP and decreased levels of haptoglobin were seen in Danish malaria patients. Consecutive studies showed that increased Hb levels were detectable 3-7 days after initiation of treatment probably because of drug induced destruction of infected erythrocytes. Increased levels of CRP were measured 0-3 days after initiation of treatment. The Hb assay was used in an epidemiological study of malaria in an area of Sudan with unstable malaria transmission. The proportion of Sudanese adults with detectable soluble Hb was higher in the rainy season with malaria transmission compared to the dry season. Hb levels in the rainy season were negatively associated with levels of haptoglobin. Most adults had increased levels of soluble Hb and decreased levels of haptoglobin 7 and 30 days after their treatment of P. falciparum malaria compared to the levels during acute disease. Thus, both soluble Hb and haptoglobin appear to be markers of recent P. falciparum infections. Very high levels of CRP protein were measured in some of the malaria patients at the day of treatment while lower levels were recorded 7 and 30 days after treatment. Soluble Hb levels were associated with malariometric parameters in a similar fashion to haptoglobin. The new Mab-based assay for measuring soluble Hb in the peripheral blood of malaria patients may be useful for future epidemiological studies of malaria.