Temperature-sensitive mutations of bacteriophage T4 gene 42 produce diverse effects upon spontaneous mutation rates. G:C → A:T transition rates are increased, often strongly; frameshift mutation rates are weakly increased; A:T → G:C transition rates (and perhaps also A:T → Py:Pu transversion rates) are decreased; and one G:C → Py:Pu transversion rate is also decreased. These results, together with certain interactions between gene-42 mutator effects and both base analogue mutagenesis and the viral error-prone repair system, suggest that the dHMC hydroxymethylase coded by gene 42 affects mutation rates in a more complex manner than by the simple regulation of the concentration of the DNA precursor dHMCTP.