Supplementation of culture medium with hemoglobin solution (Erythrogen(TM)) promoted mitotic division, cell colony formation, and plant regeneration from rice (Oryza sativa L. cv. Taipei 309) protoplasts. The mean (+/- s.e.m., n = 5) final protoplast plating efficiency (FPE) at 28 days following exposure to 1:50 (v:v) Erythrogen(TM) (0.55 +/- 0.06%) was significantly greater (P < 0.01) than in untreated controls (0.26 +/- 0.02%). A similar, but less pronounced effect (P < 0.01) also occurred with 1:100 (v:v) (PE 0.53 +/- 0.04%) and 1:500 (v:v) (PE 0.46 +/- 0.05%) Erythrogen(TM), respectively. In contrast, there was no corresponding increase in plating efficiency with Erythrogen(TM) at 1:1,000 (v:v). These beneficial effects were sustained throughout culture. This leads to a 44% increase (P < 0.01) in protoplast-derived calli producing shoots compared to controls. Cytological analyses confirmed the diploid status of plants regenerated from Erythrogen(TM)-treated and control protoplasts.