The effects of ten used drilling muds on coral health have been examined by monitoring changes in calcification rate and soluble tissue protein in the coral Acropora cervicornis. Exposure to 25-ppm (v/v) of one mud for 24 h reduced calcification rate in the growing tips by as much as 63%. Soluble tissue protein concentration dropped significantly in the growing tip after 24 h exposure to a solution of 25-, 50-, 100-, and 500-ppm of the same mud. Extensive zooxanthellae loss was visibly observed after exposure to the 500-ppm solution. Equivalent concentrations of kaolin (to produce turbidity) caused a much lower drop in calcification rate suggesting that the toxic effects of the drilling mud used were not caused by an increase in turbidity alone. The significant drop in protein concentration suggests that the use of protein or other tissue components for normalization in corals may not be justified in some cases and should be viewed with caution. In recovery experiments, corals were exposed to drilling muds (and kaolin) for 24 h; some were allowed to recover in clean seawater for 48 h. After the 24 h exposure, calcification rates were significantly less than those of controls. After a 48-h recovery period, calcification rates returned to control levels for corals exposed to kaolin and some of the drilling muds but were still significantly below controls for other muds. The results indicate that the capacity for recovery after exposure cannot be predicted from the results of experiments on exposure only. Recovery capacity must be independently verified for all studies on the effects of short-term exposure to drilling muds.