A consecutive material of 111 untreated patients with non-Hodgkin's lymphoma was studied with respect to fraction of S-phase cells in blood mononuclear cells in relation to presence of monoclonal B cells in blood (MBCB). Fraction of S-phase cells was determined by flow cytometry and estimation of MBCB was performed by kappa:lambda analysis. The fraction of S-phase cells was significantly higher (p less than 0.001) in MBCB-positive cases (median 1.2%) than in the MBCB-negative (median 0.7%). MBCB-positive patients with S-phase values greater than or equal to 1.5% had a less favourable prognosis compared to those with less than 1.5% cells in S-phase (p = 0.01). In a Cox multiparameter analysis, advanced clinical stage, high-grade morphology and high fraction of S-phase cells in blood in MBCB-positive cases were independent, statistically significant, negative prognostic indicators. The results indicate that an elevated S-phase value in blood in non-Hodgkin's lymphoma constitutes a negative prognostic factor, probably reflecting proliferating tumour cells in blood.