BACKGROUND: Heat-treated expressed breast milk is recommended by the World Health Organization as an option to reduce vertical HIV transmission in resource-poor regions. Flash-heat (FH) is a low technology pasteurization method developed for home use, but its effect on quantity and quality of breast milk immunoglobulins is unknown. OBJECTIVE: To evaluate FH's effect on breast milk immunoglobulin levels and antigen-binding capacity. DESIGN/METHODS: Fifty HIV+ mothers in South Africa provided breast milk. Part of each sample served as an unheated control; the remainder was flash-heated. Total and antigen-specific immunoglobulin A (IgA) and immunoglobulin G (IgG) were measured by enzyme-linked immunosorbent assay. Paired t test was performed on log-transformed data. RESULTS: FH significantly decreased total IgA and IgG concentrations [geometric mean (geometric SD) 318.0 (1.9) vs. 398.2 (1.9) μg/mL and 89.1 (2.7) vs. 133.3 (2.5) μg/mL, P < 0.001 each]. Similar decreases in antiĝ€"HIV-1 gp120 IgG, antiĝ€"pneumococcal polysaccharide, and antiĝ€"poliovirus IgA occurred (P < 0.001 each). Although the latter was most affected, FH retained 66% of the antigen-binding ability. In contrast, binding capacity of IgA and IgG to influenza increased after FH (P = 0.029 and 0.025, respectively). CONCLUSIONS: Most breast milk immunoglobulin activity survives FH, suggesting flash-heated breast milk is immunologically superior to breast milk substitutes. Clinical significance of this decreased immunoglobulin activity needs evaluation in prospective trials. © 2009 by Lippincott Williams & Wilkins.